# Popular Science Monthly/Volume 65/May 1904/The Development of a New Method of Research

(1904)
The Development of a New Method of Research by George Ellery Hale

THE

POPULAR SCIENCE

MONTHLY

MAY, 1904

 THE DEVELOPMENT OF A NEW METHOD OF RESEARCH.[1]

By Professor GEORGE E. HALE,

DIRECTOR OF THE YERKES OBSERVATORY, UNIVERSITY OF CHICAGO.

IN the fruitful field of astrophysical research there are few opportunities for advance so promising as those afforded by the study of the sun. As the central body of the solar system, maintaining the planets in their orbits by the power of its attraction, and supplying light and heat to the inhabitants of the earth through its radiation, the sun is an object of special interest to every student of nature. Its appeal to the imagination may be said to be threefold in character. In the first place, because of the extraordinary nature of the phenomena on its surface, and the stupendous scale of the eruptive disturbances, which are becoming more and more frequent in the present period of solar activity, the study of the sun for the purpose of gaining an understanding of its structure is in itself a sufficiently attractive object. To some, however, whose interests are aroused more particularly by the problems of chemistry and physics than by those of astronomy, this aspect of solar research may not possess special interest. But through the development of astrophysical methods, it is precisely to the physicist and the chemist that the sun should make a special appeal. For the solar observer may be the spectator of physical and chemical experiments on a scale far transcending any that can ever be performed in the laboratory. In this enormous crucible, heated to temperatures greatly exceeding those attainable by artificial means, immense masses of luminous vapor, including most of the elements known on the earth and many not yet discovered here, may be seen undergoing changes and transformations well calculated to assist in the explanation of problems which the laboratory can not solve.

But to the philosopher and the student of nature as a whole, the sun finds its highest interest in its relationship to the great problem of stellar evolution. For the central body of our system is a star, resembling in the closest way many of the stars of the sidereal universe, but possessing the unique distinction of comparative proximity to the earth. Even in the most powerful telescopes, all the other stars appear as minute points of light, which every improvement in telescope construction tends to render more minute and microscopic, so great is the distance of these stars from the observer on the earth. We have no reason to believe that telescopes will ever be constructed so powerful as to magnify a stellar image into an actual disk. With our present knowledge we therefore may not expect that the great flames and other evidences of eruptive phenomena, which we believe from inference to be as characteristic of the stars as of the sun, will ever become visible. We must therefore depend for a knowledge of such phenomena upon the one star whose surface can be studied in detail. Armed with this knowledge, we may trace out with the spectroscope successive. steps in the development of a star, from its origin in a nebula, on through the earlier stages typified by such stars as Sirius, to the condition attained in the sun. In this object we seem to observe the culmination of stellar life. For evidences of decay we must investigate the red stars, in which the radiation of heat throughout immense periods of time has resulted in cooling toward the point of final extinction. Thus we may untangle the great problem of stellar evolution, and at the same time build up a complete history of the sun, learning what it has been, what it is and what it will become.

Prior to the middle of the nineteenth century, at periods of total eclipse, when the dark body of the moon cuts off completely the bright light of the solar disk, red flames were observed at many points on the moon's circumference. At first their nature was so little understood that they were described by some observers as lunar mountains. But in 1868, through the use of the spectroscope, their true gaseous nature and their connection with the sun became known. It was found that immense masses of hydrogen and helium gas rise from a sea of flame (the chromosphere) which completely envelops the sun, and that these 'prominences' sometimes attain elevations of hundreds of thousands of miles.

The rarity and brief duration of total eclipses would have limited greatly our knowledge of the prominences, had not a method been devised by which they can be observed on any clear day in spite of the glare of our atmosphere around the sun. The instrument which permits this result to be accomplished is the spectroscope, used in conjunction with the telescope. The principle of the method is simple and easily understood. The white light of the sky, when passed through a spectroscope, is drawn out into a long rainbow band, and thereby enormously reduced in intensity. The light of the prominences, on the contrary, is concentrated in the radiations characteristic of hydrogen and helium gas, and the great dispersing power of the spectroscope merely separates more and more widely the colored images which correspond to these radiations, without greatly reducing their intensity. With the spectroscope they therefore become visible, since their images are brighter than the highly dispersed background of skylight on which they lie.

Armed with this method, observers in various parts of the world have systematically observed the forms of the solar flames on every clear day, giving us a continuous record of these phenomena now extending back for more than thirty years. From a study of this record many conclusions regarding the nature of the flames and their bearing on the question of the solar constitution, have already been reached. But the process of observation is not only slow and painstaking; it is also subject to the errors and uncertainties that attend the hand delineation of every object, seen through a fluctuating atmosphere, under unfavorable conditions. It was principally in the hope of simplifying this process, and of rendering it more rapid and more accurate, that the spectroheliograph was devised by the writer in 1889.

The principle of this instrument is very simple. Its object is to build up on a photographic plate a picture of the solar flames, by recording side by side images of the bright spectral lines which characterize the luminous gases. To accomplish this an image of the sun is formed by the telescope on the slit of a spectroscope. The light of the sun, after transmission through the spectroscope, is spread out into a long band of color, crossed by lines representing the various elements. At points where the slit of the spectroscope extends out beyond the sun's edge across a gaseous prominence, the bright lines of hydrogen and helium may be seen extending from the base of the prominence to its highest point. If a series of images of such a line, corresponding to different positions of the slit on the prominence, were recorded side by side on a photographic plate, it is obvious that they would give a representation of the form of the prominence itself. To produce such an effect it is only necessary to cause the solar image to move at a uniform rate across the first slit of the spectroscope and then, with the aid of a second slit, which takes the place of the eyepiece of the spectroscope, to isolate one of the lines, permitting the light from this line, and from no other portion of the spectrum, to pass through the second slit to a photographic plate. If the plate is moved at the speed with which the solar image passes across the first slit, an image of the prominence will be recorded upon the plate. The principle of the instrument thus lies in photographing the solar

Fig. 1.—H and K Lines on the Disk, in the Chromosphere, and in a Prominence (a).

flame through a narrow slit, from which all light is excluded except that which is characteristic of the prominence itself. This method, when tried by the writer at the Harvard Observatory in 1890, proved unsuccessful. The lack of success was partly due to the fact that a line of hydrogen was employed. This line, though fairly suitable for the photography of prominences with the perfected spectroheliograph of the present day, was too faint for successful use

Fig. 2. The Solar Chromosphere and Prominences.

amidst the difficulties which surrounded the first experiments. Accordingly, when the work was resumed a year later at the Kenwood Observatory in Chicago, an attempt was first made, through a photographic investigation of the violet and ultra-violet regions of the prominence spectrum, to discover other lines better fitted for future experiments. In the extreme violet region, in the midst of two broad dark bands which form the most striking feature of the solar spectrum, two bright lines (H and K) were found which were attributed to the vapor of calcium. They had previously been seen visually in the prominences, but on account of the insensitiveness of the eye for light of this color, their true importance had hardly been realized. A careful study soon showed them to be present in every prominence observed, at elevations above the solar surface equaling or exceeding those attained by hydrogen itself (Fig. 1, a). Their suitability for the purpose of prominence photography is due to several causes, among which may be mentioned their great brilliancy, their presence at the center of broad dark bands which greatly diminish the brightness of the sky spectrum, and the comparatively high sensitiveness of photographic plates for light of this color.

While fairly efficient from an optical point of view, the spectroheliograph of the preceding year had possessed many mechanical defects. In a new instrument, devised for use with the twelve-inch Kenwood telescope, these were overcome, and means of securing the necessary conditions of the experiment were provided. The first trials

Fig. 3. Eruptive Prominence of March 25, 1895. a, 10h 34m. Height, 135,000 Miles. b, 10h 58m. Height, 281,000 Miles.

of the instrument, made in January, 1892, were entirely successful, and the chromosphere and prominences surrounding the sun's disk were easily and rapidly recorded (Fig. 2). The details of their structure were shown with the sharpness and precision characteristic of the best eclipse photographs. And the opportunity for making such records, previously limited to the brief duration, never exceeding seven minutes, of a total solar eclipse, was at once indefinitely extended. Thus it became possible to study photographically the slowly varying forms of the quiescent, cloud-like prominences, and, to particular advantage, the rapid changes of such a violent eruption as is illustrated in Fig. 3.

But even before this primary purpose of the work had been accomplished, the possibility of making another and much more important application of the instrument' had presented itself. A photographic study of the spectrum of various portions of the sun's surface had shown the existence at many points of great regions of calcium vapor, luminous enough to render their existence evident through the production of bright H and K lines on the solar disk (Fig. 1, b and c). Some of these calcium regions had indeed been known to exist through the visual observations of Professor Young, who had observed the bright lines in the vicinity of sun-spots. But the vast extent of the calcium regions, and the characteristic forms of the phenomena, could not be ascertained by such means. What was required was such a

Fig. 4. Ordinary Photograph of the Sun.

representation of the solar disk as the spectroheliograph had been designed to give in the case of the prominences. From a consideration of the results obtained in the spectroscopic study of the disk, it appeared probable that an important application of the spectroheliograph might be made in this new direction.

Before describing this second application of the instrument, it may be well to call attention to the appearance of the sun when seen with a telescope, or when photographed in the ordinary manner, without a spectroheliograph. Such a photograph is reproduced in Fig. 4. Its most conspicuous features are the numerous dark spots scattered over the sun's surface; these are the well known sun-spots. Near the edge of the sun there may be seen certain bright regions, which are known as faculæ. The calcium regions above referred to are usually associated with the faculæ, but they lie above them, and they give no trace of their existence on ordinary photographs, like the one in Fig. 1, or to the eye when observing the sun through a telescope.

The results of the first experiments, which were made at the beginning of 1892, were such as to justify fully the expectations that had been entertained. It was at once found possible to record the forms, not only of the brilliant clouds of calcium vapor associated with the faculæ, and occurring in the vicinity of sun-spots, but also of a reticulated structure extending over the entire surface of the sun. The earliest applications of the method were made in the study of the great sun-spot of February, 1892, which, through the great scale of the phenomena it exhibited, and the rapid changes that resulted from its exceptional activity, afforded the very conditions required to bring out the peculiar advantages of the spectroheliograph. In the systematic use of the instrument continued at the Kenwood Observatory through the following years, a great variety of solar phenomena were recorded, and the changes which they underwent from day to day—sometimes, in the more violent eruptions, from minute to minute—were registered in permanent form for careful study. During this period, which ended with the transfer of the Kenwood instruments to the Yerkes Observatory, over 3,000 photographs of solar phenomena were secured. From a systematic study of these negatives, in the course of which the heliographic latitude and longitude of the calcium regions in many parts of the sun's disk were measured from day to day, a new determination of the rate of the solar rotation in various latitudes has been made. This shows that the calcium regions, like the sun-spots, complete a rotation in much shorter time at the solar equator than at points nearer the poles. In other words, the sun does not rotate as a solid body would do, but rather like a ball of vapor, subject to laws which are not yet understood.

In this first period of its career the spectroheliograph had therefore permitted the accomplishment of two principal objects. It had provided a simple and accurate means of photographing the solar prominences in full sunlight, which gave results hardly inferior to those obtained during the brief moments of a total eclipse. It had also given a means of recording a new class of phenomena, known previously to exist only through glimpses of the bright calcium lines in the vicinity of sunspots, but wholly invisible to observation either visually or on photographs taken by ordinary methods. It was not difficult to see, however, that the possibilities of the new method were much greater than had been indicated by the work so far accomplished. It seemed certain that our knowledge of the finer details of the calcium clouds would be greatly increased if provision could be made for photographing a much larger solar imago with a spectroheliograph of improved design. And

Fig. 5.—The Rumford Spectroheliograph attached to the Yerkes Telescope.

it was furthermore evident that other applications of the instrument, involving the use of different spectral lines, and the employment of principles which had not entered in the Kenwood work, might reasonably be hoped for.

The construction of the great forty-inch telescope of the Yerkes Observatory provided the first requirement of this new work, namely, a large solar image, having a diameter of seven inches as compared with the two-inch image given by the Kenwood telescope. The construction of a spectroheliograph large enough to photograph such an image of the sun involved serious difficulties, but these were finally overcome. The Rumford spectroheliograph, designed to meet the special conditions of the new work, was constructed in the instrument

Fig. 6.—The Sun showing the Calcium Flocculi (H2 Level). 1903, August 12, 8h 52m. C.S.T.

shop of the Yerkes Observatory, and is now in daily use with the forty inch telescope (Fig. 5).

In this instrument the solar image is caused to move across the first slit by means of an electric motor, which gives the entire telescope a slow and uniform motion in declination. The sun's light, after passing through the first slit, is rendered parallel by a large lens at the lower end of the collimator tube. The parallel rays from this lens fall upon a silvered glass mirror, from which they are reflected to the first of two prisms, by which they are dispersed into a spectrum. After passing through the prisms, the light, which has now been deflected through an angle of 180°, falls upon a second large lens at the

Fig. 7.—Minute Structure of the Calcium Flocculi at H2 Level. 1903, September 22. (Scale: Sun's Diameter ${\displaystyle =}$ 0.890 Meter.)

lower end of the camera tube. This forms an image of the spectrum at the upper end of the tube, where the second slit is placed. Any line in the spectrum may be made to fall upon this slit by properly adjusting the mirror and prisms. Above the slit, and nearly in contact with it, the photographic plate is mounted in a carriage which runs on tracks at right angles to the length of the slit. The tracks are covered by a light-tight camera box, so that no light can reach the plate except that which passes through the second slit. While the solar image is moving across the first slit, the plate is moved at the same rate across the second slit, by a shaft leading clown the tube from the electric motor, and connected, by means of belting, with screws that drive the plate-carriage.

Photographs of the solar disk taken with this instrument under good atmospheric conditions show a multiplicity of fine details of which no trace appears on the Kenwood plates (Fig. 6). The entire surface of the sun is shown by these plates to be dotted over with minute luminous clouds of calcium vapor, separated by dark spaces, and closely resembling in appearance the well-known granulation of the solar photosphere (Fig. 7). A sharp distinction must, however, be drawn between this appearance, which is wholly invisible to the eye at the telescope, and the granulation of the photosphere. In accordance with Langley's view the grains into which the surface of the sun is resolved under good conditions of visual observation are the extremities of columns of vapor rising from the sun's interior. They seem to mark the regions at which convection currents, proceeding from within the sun, bring up highly heated vapors to a height where the temperature becomes low enough to permit them to condense. It might be anticipated that out of the summits of these condensed columns, other vapors, less easily condensed, might continue to rise, and that the granulated appearance obtained with the spectroheliograph may represent the calcium clouds at the summits of these columns. We might indeed go a step further, and imagine the larger and higher calcium clouds to be constituted of similar vaporous columns, passing upward through the chromosphere and perhaps at times extending into the prominences themselves. But without a means of research now to be described, which represents another application of the spectroheliograph, involving a new principle, the true nature of these phenomena could not be ascertained.

Mention has already been made of the luminous faculæ, which are simply regions in the photosphere that rise above the ordinary level. At the edge of the sun their summits lie above the lower and denser part of that absorbing atmosphere which so greatly reduces the sun's light near the limb, and in this region the faculæ may be seen visually. At times they may be traced to considerable distances from the sun's limb, but as a rule they are inconspicuous or wholly invisible toward the central part of the solar disk. The Kenwood experiments had shown that the calcium vapor usually coincides closely in form and position with the faculæ, and hence the calcium clouds were long spoken of under this name. In the new work at the Yerkes Observatory the distinction between the calcium clouds and the underlying faculæ is so marked that a distinctive name for the vaporous clouds has become necessary. They have therefore been designated flocculi, a name chosen without reference to their actual nature, but suggested by the flocculent appearance of the photographs.

In order to analyze these flocculi and to determine their true structure, a method was desired which would permit sections of them at different heights above the photosphere to be cut off, as it were, and

Fig. 8.—H and K Lines in Electric Arc, showing Reversals.

photographed. Fortunately there is a simple means of accomplishing this apparently difficult object. At the base of the flocculi the calcium vapor, just rising from the sun's interior, is comparatively dense. As it passes upward through the flocculi it reaches a region of much lower pressure, and during the ascent it might be expected to expand and therefore to become less dense. Now we know from experiments in the laboratory that dense calcium vapor produces very broad spectral bands, and that as the density of the vapor is decreased these bands narrow down into fine sharp lines (Fig. 8). An examination of the solar spectrum will show that the H and A lines of calcium give evidence of the occurrence of this substance under widely different densities in the sun. The broad dark bands, which for convenience we designate as H1 and K1, are due to the low-lying dense calcium vapor (Fig. 1). At their middle points (over flocculi) are seen two bright lines, which are much narrower and better defined. These lines, which We designate as H2 and K2, are the ones habitually employed in photographing the flocculi with the spectroheliograph. Superposed upon these bright lines are still narrower dark lines, due to the absorption of cooler calcium vapor at higher elevations (H3, K3). It will be seen that the evidence of the existence of calcium vapor at various densities in the sun is complete, and that we may here find a way of photographing the vapor at low levels without admitting to the photographic plate any light that comes from the rarer vapors at higher levels. It is simply necessary to set the second slit of the spectroheliograph near the edge of the broad H1 or K1 bands, in order to obtain a picture showing only that vapor which is dense enough to produce a band of width sufficient to reach this position of the slit. No light from the rarer vapors above can enter the second slit under these circumstances, since they are incapable of producing a band of the necessary breadth. Light from the denser vapors below will, however, enter the slit. But since it happens, within certain limits, that the vapor grows brighter and also expands as it rises above the photosphere, it seems to follow that a photograph will generally represent a section of the vapor at the level corresponding to the position of the line on the slit, and that little confusion will result from the presence of the denser but less brilliant vapors lying below.

The great sun-spot of October, 1903, afforded an opportunity to try this method in a very satisfactory manner. Sections of the calcium vapor in the neighborhood of this spot-group, corresponding to the two different levels photographed on October 9, are shown in Fig. 9.[2] The manner in which the vapor at the H2 level overhangs the edge of the sun-spot is very striking, and thorough study should throw some light on the conditions which exist in such regions. For it is possible, not only to photograph sections of the vapor at various levels, but also to ascertain by the displacement of the H2 line, as photographed with a powerful spectroscope, the direction and velocity of motion of the vapor which constitutes the flocculus. It is commonly found that the vapor is moving upward at a velocity of about one kilometer per second, though the velocity varies considerably at different points and under different conditions. In the near future it is proposed to make a careful systematic study of the motion of the vapor at various regions in the flocculi, in order to determine whether it is generally

 Oct 9, 3h 43m. Calcium Flocculi, Middle H1 Level. October 9, 3h 30m. Calcium Flocculi H2 Level. Slit at λ 3966 Slit at λ 3968.6 Fig.9.—The Great Sun-Spot of October 1908.For comparison with Stereoscope (See Footnote on p. 494).

upward or whether there may be frequent evidence of downward currents or of currents more nearly parallel to the solar surface. Many photographs show the existence of flocculi remarkable for their great brilliancy. In these regions active eruptions are in progress. The vapor, rendered highly luminous by intense heat or other causes, is shot out from the sun's interior with great velocity. Consequently there are rapid changes in the forms of these brilliant regions, whereas the more extensive flocculi, which occupy the greater part of the photograph, change slowly, and represent a much less highly disturbed condition of affairs. The brilliant eruptive flocculi always occur in active regions of the solar surface, and doubtless correspond with the eruptive prominences sometimes photographed projecting from the sun's limb. A remarkable instance was recorded on the Kenwood photographs, which showed four successive stages in an eruption of calcium vapor on an enormous scale. A vast cloud thrown out from the sun's interior completely blotted from view a large sunspot, and spread out in a few minutes so as to cover an area of four hundred millions of square miles.

As already remarked, these eruptive flocculi probably correspond in many instances with the eruptive prominences observed at the sun's edge. But it must not therefore be concluded that the quiescent calcium flocculi correspond with the quiescent, cloud-like prominences. As a matter of fact, we have good evidence for the belief that the flocculi shown in these photographs represent in most instances comparatively low-lying vapors, which, if observed at the sun's edge, would hardly project appreciably above the level of the chromosphere.

In a few cases, represented, perhaps, by the dark calcium flocculi found on certain photographs, the quiescent calcium prominences have been recorded in projection on the disk. But such instances will remain exceptional until a spectroheliograph has been constructed of such high dispersion as to permit photographs to be made with the light of the K3 line exclusively.

So far we have considered the photography of the sun with the light of the H and K lines of calcium. But it must naturally occur to any one familiar with the solar spectrum that it should be possible to take photographs corresponding to other lines, and thus representing the vapors of other substances. In the solar spectrum some 20,000 lines have been recorded in the great photographic map and catalogue of Rowland, representing almost all the elements known on the earth, and doubtless including many of the radiations of substances which are as yet unknown to terrestrial chemistry. Just as the gas helium was discovered in the sun long before it was round by Ramsay in the laboratory, so we may confidently expect that many other substances represented by lines in the solar spectrum will ultimately be detected on the earth.

Now these lines, like the H1 and A1 hands, are dark, and at first sight it might be supposed that for this reason the vapors corresponding with them could not he photographed with the spectroheliograph. But a moment's consideration will show that no serious difficulty need arise from this cause. For the darkness of the lines is only relative; if they could be seen apart from the bright background of continuous spectrum on which they Lie, these lines would shine with great brilliancy. It is thus evident that if all light except that which comes from one of these dark lines can be excluded from the photographic plate by means of the second slit of the spectroheliograph, it should he possible to obtain a photograph showing the distribution of the vapors corresponding with the line in question.

At this point attention should he called to the extreme sensitiveness of the spectroheliograph in recording minute variations in the intensity of a line—variations so slight that no trace of them can be seen in a spectrum photograph showing only the line itself. A well-known physiological action is here concerned, for it is common experience that the eye can not detect minute differences of intensity in various parts of an extremely narrow line, whereas these would become much more conspicuous if the line were widened out into a band of considerable width. The action of the spectroheliograph is to record side by side upon the photographic plate a great number of images of a line which, taken together, build up the form of the region from which the light proceeds. In this way the full benefit of the physiological principle is derived, and very minute differences of intensity between various parts of the solar disk are clearly registered upon the photographic plate.

It is obviously essential in photographing with the dark lines to exclude completely the light from the continuous spectrum on either side of the line employed. The admission of even a small quantity of this light might completely nullify the slight differences of intensity recorded by the aid of the comparatively faint light of the dark line. As the second slit can not be narrow r ed beyond a certain point, it is evident that for successful photography with the dark lines their width must be increased by dispersion in the spectroheliograph to such a degree as to make them wider than the second slit.

The first successful photographs obtained with dark lines were made with the Rumford spectroheliograph in May, 1903. The lines of hydrogen were chosen for this purpose, on account of their considerable breadth, and because of the prominent part played by this gas in the chromosphere and prominences. In order to secure sufficient width of the lines, the mirror of the spectroheliograph was replaced by a large plane grating having 20,000 lines to the inch. After leaving the grating the diffracted light enters the prisms, where it is still further dispersed before the image of the spectrum is formed upon the second slit. The effect of the prisms is not only to give additional dispersion, but also to reduce the intensity of the diffuse light from the grating —a most important matter in work of this nature. The hydrogen lines employed were , and ; H\$ is perhaps the best of these three lines for the purpose.

On developing the first plate we were surprised to find evidences of a mottled structure covering the sun's disk, resembling in a general way the structure of the calcium flocculi, but differing in the important fact that whereas the calcium flocculi are bright those of hydrogen are dark. This result was confirmed by subsequent photographs, and it was found that in general the hydrogen flocculi are dark, although in certain disturbed regions bright hydrogen flocculi appear. Some of these are eruptive in character, and correspond closely with the brilliant eruptive calcium flocculi. But in other cases, in regions where no violent eruptive disturbances seem to be present, the hydrogen flocculi sometimes appear to be bright instead of dark. Such regions are usually in the immediate vicinity of active sun-spots, where it is probable that the temperature of the hydrogen vapor is considerably higher than in the surrounding regions. The spectroheliograph thus seems to afford a method of distinguishing between regions of higher and lower temperatures—an additional property which should prove of great value in investigations on the vapors associated with sun-spots. It is possible, of course, that the increased brightness is due not merely to an increase of temperature, but to other causes, perhaps of a chemical or electrical nature, which are not yet understood. But in any event, the method serves to differentiate these regions from others in which these conditions are not fulfilled, and the possibility of making such a differentiation is of value, even if we do not as yet understand the actual cause of the increased brightness.

The comparative darkness of the hydrogen flocculi evidently indicates that this gas in the flocculi for some reason radiates less light than the hydrogen gas which, probably after diffusing from the flocculi, has spread in a nearly uniform mass over the entire surface of the sun. For the present the simplest hypothesis is to assume that the diminished brightness of the flocculi is due to a lower temperature at these points, perhaps caused by the rapid expansion of the gas as it rises from the interior of the sun into the region of greatly reduced pressure above the chromosphere. On such an assumption it would seem probable that the hydrogen flocculi really represent the hydrogen prominences, which lie at a considerable height above the chromosphere, in a region of very low pressure, where the effect of expansion should have produced the greatest fall in temperature. It may ultimately appear that some other explanation must be adopted, especially since the hydrogen in the upper part of the chromosphere seems to be represented by the smaller hydrogen flocculi which form a network over the entire surface of the sun. It already seems probable, in spite of the comparatively small width of the hydrogen lines, that it will become possible to secure photographs corresponding to different elevations,

 N E Fig. 10.—3h 57m. Calcium Flocculi, K2 Level. Slit at λ 3933 8. W S Fig. 11.—11h 0m. Hydrogen Flocculi. Slit at Center of Hy. (Bright Eruptive Flocculi West of Spot.) Hydrogen and Calcium Flocculi, 1903, July 7.(Scale: Sun's Diameter's ${\displaystyle =}$ 0.290 Meter. )

just as has been done for calcium with the broad H and K bands. This would permit the lower regions to be differentiated from the higher ones, and thus assist toward an understanding of the true cause of the apparent darkness. A satisfactory comparison of the forms of the hydrogen flocculi with those of the calcium flocculi can only he made after the question of level has been solved. Although there is a general resemblance in form, as may be seen by reference to Figs. 10 and 11, the differences are nevertheless striking enough to suggest that various researches, interesting on physical and chemical grounds, should be undertaken in the future. For instance, a series of simultaneous photographs, in both hydrogen and calcium lines, taken at brief intervals during the course of a violent eruption, might show interesting peculiaritles in the relative forms of the hydrogen and calcium flocculi, corresponding to different velocities and distribution of the respective gases.

The Rumford spectroheliograph has also been used to secure photographs with some of the stronger dark lines of iron and other substances, which show the vapors of these metals on the sun. But even with the grating, the dispersion is insufficient to give thoroughly trustworthy results, except in a very few cases. It is evident that much greater dispersion must be employed if the full capacity of the method is to be brought out in future work.

It is perhaps worth while to consider what are the logical steps to be taken in the future development of the spectroheliograph. As at present used, it is capable of solving a large number of problems if employed systematically to register the changing forms of the calcium and hydrogen flocculi. The method of photographing sections at different levels, and the method of detecting local differences of temperature or of physical or chemical state, should also permit important knowledge to be gained. But to stop at the point reached, when there is so much of promise in the further perfection of the method, would commend itself to no one interested in the advancement of research. The principal requirements of an instrumental nature are:

1. Greatly increased dispersion in the spectroheliograph, through the use of prisms or gratings in conjunction with collimator and camera lenses of considerable focal length.

2. A focal image of the sun at least twenty inches in diameter, of which zones at least four inches wide can be photographed in monochromatic light.

These considerations would point to the use of spectroheliographs, of from 12 to 40 feet focal length, provided with large gratings or with three or four prisms. Such instruments would necessarily be mounted in fixed positions on massive piers. A solar image 20 inches in diameter would involve the use of a telescope about 180 feet long, and the importance of providing tor simultaneous photography in several lines at different parts of the spectrum, would require that the sun's image be formed by mirrors instead of lenses. Thus the telescope should be a horizontal reflector of the cœlostat type. The aperture of the mirrors should be as great as possible, since the high dispersion of the spectroheliograph, even in the most favorable cases, will involve long exposures.

But the most important requisite is such a condition of the atmosphere as will give the finest possible definition of the solar image. This would involve the establishment of the instruments at some particularly favorable site, where careful telescopic tests have shown the definition of the solar image to be exceptionally tine. Such a site is most likely he found in regions where the sky remains cloudless for weeks at a time: a point of great importance, since at such a place the various phases of changing phenomena could be followed up day after day with the same instruments. A still more thorough study of constantly varying solar phenomena could be secured through the cooperation of a chain of suitably equipped observatories, so distributed in longitude as to permit the sun to be kept continuously under observation.

With such large spectroheliographs, employed with a well-defined solar image of sufficient size, it should be possible to study not only the vapors lying around and above sun-spots, but those which constitute the spot itself. With sufficiently high dispersion, for example, it ought to be possible to secure photographs with the slit set at different points on some of the broadest of the 'widened' lines, giving sections of the vapors of the dark umbra of the spot at different depths below the surface. A comparative study of the forms of the umbra, as recorded in different widened lines, and of those bright forms which must appear on photographs taken with Fraunhofer lines that are weakened in intensity or transformed into bright lines in sun-spots, should provide data for the solution of important questions relative to the solar constitution.

But the spectroheliograph, though promising to supply an exceedingly powerful method of attack, is only one of many instruments required in a comprehensive investigation of solar phenomena. Powerful spectroscopes, equaling or surpassing in resolving power the largest instruments now employed in the physical laboratory, must be used simultaneously with the spectroheliograph in a study of the various vapors. In such a research the displacements of lines in various regions, corresponding to the effect of pressure or to the motion of the gases in the line of sight, would play a prominent part. This most precise quantitative work must lie accompanied by a systematic record, extending through many sun-spot periods, of the lines which are widened in sun-spots. Furthermore, there should be accurate measurements, with the bolometer or radiometer, of the heat radiated from different parts of sun-spots and from other regions of the sun's surface. Visual studies of the solar details under the best atmospheric conditions, and direct photographs made in the ordinary way, will also be essential. From such a mass of observations, if systematically made and studied, a considerable increase in our knowledge of the solar constitution might reasonably be expected to follow.

It would be beyond the province of my immediate subject to discuss the methods by which the study of the physical constitution of the nebulæ and stars may be expected to throw light on the past and future of the sun. But I can not refrain from remarking that through recent improvements in reflecting telescopes, and through the further improvements which are promised in the immediate future, a great advance in this department of astrophysical research may confidently be expected. It thus appears that if the powerful instruments required for these investigations can be provided, the opportunity should exist during the next quarter of a century to make important additions to our knowledge of the origin and development of the sun, and at the same time to throw new light on the great problem of stellar evolution.

1. Address delivered on November 23, 1903, before the University of Chicago Chapter of the Society of Sigma Xi.
2. Although these photographs have been arranged for comparison with the stereoscope, it is to be understood that no stereoscopic effect in the ordinary sense will be obtained in examining them. The purpose of using the stereoscope is simply to allow the images to be superposed, thus permitting them to be seen at the same point in rapid succession by simply moving a card so as to cover alternately the two lenses of the stereoscope. In this way the sun-spot may be examined, first as it appears at the low level of the denser vapor and then as it appears at the higher level of the rarer vapor. Thus the manner in which the calcium flocculi overhang the penumbra, and sometimes the umbra, of spots can be observed.