Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/876

840 P H O P H O sequently worked at the same idea. It met, however, with no very great success. The introduction of dry plates was a great step for the landscape photographer, as it enabled him to carry a supply of plates in the field, and to develop them at home. To economize space aud weight, what are known as " double backs " were in vented. A " double back " is a dark slide in which two plates are placed back to back, being separated by an opaque plate. Each side of the slide can be drawn up or out so as to expose each plate. What are known as changing boxes answer the same purpose. They hold from one to two dozen plates, and by means of a special arrangement each plate can be conveyed to or removed from the dark slide without exposure to light. There are other plans also by which a certain number of plates can be carried in trie camera itself and exposed in succession. The writer s opinion of such instruments is that they possess no striking advantage and many disadvantages, unless for very special purposes. Even for a minia ture camera for taking instantaneous street views whilst holding the apparatus in the hand the use of double backs is to be preferred. An excellent specimen is a camera made by Marion & Co. of London (see fig. 9) : it is entirely of metal, and iitted with a iinder and in stantaneous shutter, one which should stand any amount of rough usage. The whole apparatus, including a dozen plates, can easily be carried in the pocket. The dark slides are strongly made of metal. In the preceding sketch, brief though it is, of the successive improvements in cameras, probably enough has been said to show the very remarkable development that has taken place since the days when a cigar-box and spectacle leiis were used to obtain an image on a sensitive plate. (W. DE W. A. ) PHOTOMETRY, CELESTIAL. The earliest records that have come down to us regarding the relative positions of the stars in the heavens have always been accompanied with estimations of their relative brightness. With this brightness was naturally associated the thought of the relative magnitudes of the luminous bodies from whence the light was assumed to proceed. Hence in the grand catalogue of stars published by Ptolemy (c. 150 A.D.), but which had probably been formed three hundred years before his day by Hipparchus, the 1200 stars readily visible to the naked eye at Alexandria were divided into six classes according to their lustre, though instead of that term he uses the word ;ueye#os or " magnitude " ; the brightest he designates as being of the first magnitude, and so down wards till he comes to the minimum visibile, to which he assigns the sixth. These magnitudes he still further divides each into three. To those stars which, though ranged in any particular order of brightness, nevertheless exceed the average of that order in lustre he attaches the letter /*, the initial letter in peifov (greater), and to those in the same order which exhibit a lustre inferior to that of the average he affixes the letter e, the initial letter of e Atto-o-wv. With this sort of subdivision he passes through all the six orders of magnitude. He does not, indeed, tell us the precise process by which these divisions were esti mated, but the principle involved is obvious. The eye was here made the natural photometer, and it is certain that even in the instances where modern instrumental ap pliances are called into requisition the ultimate appeal is made to perception by the eye. Moreover, it is one of the many remarkable instances of the acuteness and precision of the Greek mind that for upwards of 1500 years no real improvement was made in these estimations of lustre by any of Ptolemy s numerous successors in this field of re search. Flamsteed was the first astronomer who extended the estimation of magnitude to stars visible only by the telescope, and he improved Ptolemy s notation by writing 4 3 instead of 5, /JL indicating thereby an order of mag nitude brighter than the average of a fourth, but inferior to that of a third and 3*4 for 8, e, and so on. Later astronomers have sometimes adopted a more precise nomen clature by subdividing the several orders decimally, but it does not appear that by any immediate and unaided effort the eye can estimate subdivisions of lustre exceeding the thirds adopted by the Greek philosopher. It was not till the year 1796 that any real advance was made in stellar photometry. Sir W. Herschel, instead of assigning a particular magnitude to stars, arranged them in small groups of three or four or five, indicating the order in which they differed from each other in lustre at the time of observation. This method was admirably adapted to the discovery of any variations in brightness which might occur in the lapse of time among the members of the group. Sir William observed in this way some 1400 stars, pub lished in catalogues scattered through the Philosophical Transactions from 1796 to 1799; but he discontinued the work before its conclusion. It rni^ht be umed that such a work touches on no human interests, but it rightly seemed otherwise to the philosophic mind of the great astronomer. He remarked that the sun is, after all, only one among the stars, and that what befalls them in the way of varying light as time proceeds may also befall the sun. He puts the question, " Who would not wish to know what degree of permanency we ought to ascribe to the lustre of our sun? Not only the stability of our climates, but the very exist ence of the whole animal and vegetable creation itself, is involved in the question. Where can we hope to receive information upon the subject but from astronomical observa tions I" 1 These researches of the elder Herschel were in due time followed by those of his son, Sir John, about the year 1836 at the Cape of Good Hope. He both extended and improved the methods adopted by his father at Slough, and by a method of estimated sequences of magnitude he hoped to arrange all the stars visible to the naked eye at the Cape or in England in the order of their relative lustre, and then to reduce his results into the equivalent magni tudes adopted by the universal consent of astronomers. Sir John, however, like his father, left this important labour incomplete. Not only is the work one of great and con tinuous effort, but the effects of ever -varying meteoro logical conditions greatly impede it. Moreover, there is an unsatisfactory indefiniteness attending all estimations made by the unaided eye ; numerical or quantitative com parisons are out of the question, and hence we find Sir John, in the very midst of establishing his "sequences," adopting also an instrumental method which might lead him to more definite results. In the year when Sir John Herschel concluded his photometric work at the Cape (1838) Dr Argelander com menced, and in 1843 completed, his Uranometria Nova, in which the magnitudes of all stars visible to the unaided eye in central Europe are catalogued with a precision and completeness previously unknown. It contains 3256 stars, and although it will probably be superseded by instru mental photometry it must ever remain a monument of intelligent patience. Argelander s labours were confined to stars visible to the naked eye ; by the aid of his assist ants, Dr Schonfeld and Dr Kriiger, a catalogue of magni tudes and celestial coordinates was ultimately published in their well-known Durchmusterung, extending to the enormous number of 324,000 stars. Dr Gould also, in his Uranometria Argentina, has done similar work for stars visible only in the southern hemi sphere, and with the aid of his colleagues has attained to an exactness and precision in his estimations of stellar lustre certainly not hitherto surpassed. There have been other worthy labourers in the same field, each of whom has rendered efficient service, such as Dr Heis and M. Houzeau ; but it is chiefly to the labours of Argelander and Gould that astronomers at present make their appeal. It is to Sir John Herschel that we are indebted for the first successful attempt at stellar photometry by what may 1 Phil. Trans., 1796, p. 184.