Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/877

PHOTOMETRY be termed " artificial " means. By the aid of appliances of the simplest kind he deflected the light of the moon (by means of the internal reflexion of a rectangular prism) through a small lens 12 inches in diameter and of very short focus, 2253 inches, so as to form a sort of artificial star in its focus. By the instrumentality of strings and a wooden pole he could move this artificial star of compari son so as to be in the same line of sight with any actual star whose light he proposed to measure. Other strings enabled him to remove this microscopic lunar image to such a distance from the eye that its light was adjudged to be sensibly the same as that of the star compared. The dis tance of the short focused lens with the image contiguous to it was measured by a graduated tape, and the inverse squares of these distances afforded relative numerical mea sures of the brightness of the several stars thus brought into ocular juxtaposition with the equalized light of the tiny lunar image. In this way he proceeded with the ob servations of a considerable number of stars, and these, by appropriate methods, were reduced so as to afford the means of the comparison of their relative brightness when set side by side with results obtained by means of his " sequences," and with the estimated magnitudes of preceding astro nomers. Sir John, however, did not go on to the formation of a complete " uranometria." While he was thus busy at the Cape of Good Hope, Steinheil at Munich had com pleted for Dr Seidel an instrument nearly the same in principle but more manageable in form. He divided the small object-glass of a telescope into two halves, one of which was movable in the direction of its axis. The images of two stars whose light he desired to compare were formed by the intervention of prismatic reflexion, nearly in the same line of sight, and one of the lenses was then moved until the light of the two stars near the respective foci of the semi-lenses seemed equal to the judgment of the observer s eye. The distance through which it was neces sary to bring the movable lens furnished the data for com paring the relative lustre of the two stars in question. A large amount of work was thus achieved by Seidel, which for a considerable time has been, with greater or less reason, regarded as worthy of confidence in regard to precision (Trans. Mun. Acad., vol. ii.). Dr Zollner substituted the deflected and reduced image of a lamp for one of Steinheil s stars, and the intensity of this light, or artificial star, he could by means of double refraction reduce in any measur able proportion he pleased according to the well-known relations of polarized light. In this way he could equalize the light of the artificial lamp-star with that of the real star with which he compared it ; and the division of the lens was thus dispensed with, but a new difficulty was intro duced in the impossibility of maintaining the constancy of the flame. Dr Zollner also availed himself of the effects of double refraction in altering at will the colour of his artificial star of comparison. This ingenious form of photometer has enjoyed considerable reputation, but no astronomer has yet persevered in producing a complete " uranometria " by its aid. The most recent and probably the most successful device for a stellar photometer on the principle of equalizing lights is that invented by Professor Pickering of Harvard College. He deflects the light of Polaris, or of some other star such as A Ursas Minoris, by means of prismatic reflexion, and he contrives to form an image of it contiguous to the image of any other star selected on the meridian. The equalization of the lights is then effected by the intervention of a polarizing appa ratus, such as that adopted by Zollner. Thus the artificial and in many respects objectionable lamp-star of Zollner is dispensed with. Professor Pickering, with singular invent ive power, has devised many other forms of stellar photo meters on virtually the same principle ; for a detailed account of these labours the reader is referred to the Annals of the Harvard College Observatory (vol. xi.). Unlike his eminent predecessors, the American astronomer is persever ing in the formation of a complete catalogue of star- magnitudes. It has been already stated that mere estimations of relative brightness by the unaided eye are inadequate to the production of numerical quantitative results. In the instrumental devices explained, whether by means of the alteration of distances or by the known alteration of planes of polarization, no such defect exists. By their means it is possible to obtain a fairly exact numerical expression for the ratio of the intensities of the two lights measured. On applying a photometric measurement it is found that the ratio of the intensities of the lights in passing from one magnitude to the next, even in the conventional magni tudes of Argelander and Gould, is not by any means con stant, and even hardly definite. At the suggestion of Mr Pogson it is now generally accepted by astronomers that the adopted and conventional ratio of the intensity of light in passing from one magnitude to another shall be 2 "5 12, a convenient number because its logarithm is 4, which is easily remembered, and still more so because on the whole it agrees better than any other number with the varying light-ratio existing among the hitherto received orders of magnitude obtained by eye-estimation alone. There remains still another principle on which a stellar photometer may be successfully formed, and which has been recently largely applied to the determination of star- magnitudes at the university observatory, Oxford. It is constructed on the principle that the absorption of light in passing through a uniform medium depends, c&teris paribus, upon the thickness. On this principle a thin wedge is constructed of homogeneous and nearly neutral-tinted glass, through which the images of stars formed in the focus of a telescope are viewed. Simple means are con trived for measuring with great exactness the several thicknesses at which the light of these telescopic star- images is extinguished. In this way the light of any star can be readily compared with that of Polaris (or any other selected star) at the moment of observation, and thus a catalogue of star -magnitudes can be formed. This method has been already applied by Professor Pritchard to all the brighter stars north of the equator ; the results are published in the forty-seventh volume of the Memoirs of the Royal Astronomical Society, and are to be speedily followed by a complete catalogue, extending to all the stars in Argelander s Uranometria Nova north of the equator, and to a few others beyond. For the details of the processes adopted the reader must here, as in all other cases, consult the original researches. Even in a rapid sketch of so extensive a subject some notice must be taken of the application of photometry to the determination of the relative amount of light received on the earth from the sun, the moon, and the planets. The methods by which these ratios have been obtained are as simple as they are ingenious ; and for them we are mainly indebted to the labours of Bouguer and Bond. The former philosopher compared the light received from the sun with that from the moon in the following fashion in 1725. A hole one-twelfth of a Paris inch was made in the shutter of a darkened room ; close to it was placed a concave lens, and in this way an image of the sun 9 inches in diameter was received on a screen. Bouguer found that this light was equal to that of a candle viewed at 16 nches from his eye. A similar experiment was repeated with the light of the full moon. The image now formed was only two-thirds of an inch in diameter, and he found that the light of this image was comparable with that of the same candle viewed at a distance of 50 feet. From XVIII. 1 06