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370 THE PHOTOGRAPHIC NEWS. [July 31, 1868. regular occupation will be better able to fix these data than the writer, by whom photometry is only occasionally pursued as a means of scientific research. Already many improvements suggest themselves, and several causes of variation in the light have been noticed. Future experiments may point out how these sources of error are to be overcome ; but at present there is no necessity to refine our source of standard light to a greater degree of accuracy than the photometric instrument admits of. The instrument for measuring the relative intensities of the standard and other lights next demands attention. The con trivances in ordinary use are well known, Most of them depend on a well-known law in optics; namely, that the amount of light which falls upon a given surface varies inversely as the square of the distance between the source of light and the ob ject illuminated. The simplest observation which can be taken is made by placing two sources of light (say a candle and gas lamp) opposite a white screen, a few feet off, and placing a stick in front of them, so that two shadows of the stick may fall on the screen. The strongest light will cast the strongest shadow ; and by moving this light away from the stick, keeping the shadows side by side, a position will at last be found at which the two shadows appear of equal strength. By measuring the distance of each light from the screen, and squaring it, the product will give the relative intensities of the two sources of light. In practice, this plan is not sufficiently accurate to be used except for the roughest approximations; and from time to time several ingenious contrivances, all founded upon the same law, h ive been introduced by scientific men, by which a much greater accuracy is obtained ; thus in Ritchie’s photometer the lights are reflected on to a piece of oiled paper in a box, and their distances are varied until the two halves of the paper are equally illuminated. In Bunsen’s photometer, which is the one now generally used, the lights shine on opposite sides of a disc of white paper, part of which has been smeared with melted spermaceti to make it more transparent. When illuminated by a front light, the greased portion of the paper will look dark ; but if the observer goes to the other side of the paper, the greased part looks the lighter. If, therefore, lights of unequal intensity are placed on opposite sides of a piece of paper so prepared, a difterence will be observed; but by moving one backwards or forwards, so as to equalize the intensity, the whole surface of the paper will appear uniformly illuminated on both sides. This photometer has been modified by many observers. By some the disc of paper is moved, the lights remaining sta tionary ; by others the whole is enclosed in a box, and various contrivances are adopted to increase the sensitiveness of the eye and to facilitate calculation; but in all these the sensi tiveness is not greatly augmented, as the eye cannot judge of very minute differences of illumination approximating to equality. In 1833, Arago described a photometer in which the pheno mena of polarized light were employed. This instrument is fully described, with drawings, in the tenth volume of the CEuvres Completes de Francois Arago ; but the description, although voluminous, is far from clear. The principle of its construction is founded on “the law of the square of the cosines,” according to which polarized rays pass from the ordinary to the extraordinary imago. The knowledge of this law, he says, will not only prove theoretically important, but will further lead to the solution of a great number ot very important astronomical questions. Suppose, for example, that it is wished to compare the luminous intensity of that portion of the moon directly illuminated by the solar rays with that of the part which re ceives only light reflected from the earth, called the partie cendree. Were the law in question known, the way to proceed would be as follows:—After having polarized the moon’s light, pass it through a doubly refracting crystal, so disposed that the rays, not being able to bifurcate, may entirely undergo ordinary refraction. A lens placed behind this crystal will therefore show but one image of our satellite ; but as the crystal in rotating on its axis passes from its original position, the second imago will appear, and its intensity will go on augmenting. The movement of the crystal must be arrested at the moment when, in this growing extraordinary image, the segment corre sponding to the i a.-t of the moon illuminated by the sun ex hibits the intensity of the ashy part shown by the ordinary image. From these data it is easy to perceive, he says, that the problem is capable of solution. In another part of the same volume, after speaking of the polariscope which goes by his name, Arago writes ; —“ I have now arrived at the general principle upon which my photo metric method is entirely founded. The quantity (I do not say the proportion)—the quantity of completely polarized light, which forms part of a beam partially polarized by reflection, and the quantity of light polarized rectangularly, which is con tained in the beam transmitted under the same angle, are exactly equal to each other. The reflected beam and the beam transmitted under the same angle by a sheet of parallel glass have in general very dissimilar intensities; if, however, we examine with a doubly refracting crytsal, first the reflected and then the transmitted beam, the greatest difference of intensity between the ordinary and the extraordinary images will be the same in the two cases, because this difference is precisely equal to the quantity of polarized light which is mixed with the common light.” In Arago’s astronomy, the author again describes his photometer in the following words: “ I have constructed an apparatus by means of which, upon operating with the polarized image of a star, we can succeed in attenuating its intensity by degrees exactly calculable after a law which 1 have demonstrated.” It is difficult to obtain an exact idea of this instru ment from the description given; but from thedrawings itwould appear to be exceedingly complicated, and to be different in principle and con struction from the one now about to be described. The present photo meter has this in common with that of Arago, as well as with those described in 1853 by Bernard,* and in 1854 by Babinet,t that the phenomena of polarized light are used for effecting the desired end. But it is believed that the present arrangement is quite new, and it certainly appears to answer the purpose in a way which leaves little to bo desired. The in strument will be better understood if the principles on which it is based are first described. Fig. 1 shows a plan of the arrange ment of parts, not drawn to scale, and only to be regarded as an outline sketch to assist in the comprehension of general principles. Let D repre sent a source of light. This may be a white disc of porcelain or paper illuminated by any artificial or natural light. C represents a similar white disc, likewise illuminated. It is required to compare the photometric in tensities of D and C. (It is necessary that neither Dnorb should contain any polarized light, but that the light coining from them, represented on each disc by the two lines at right angles to each other, forming a cross, should bo entirely un polarized.) Let H represent a double refracting achromati prism of Iceland spar; this will resolve the disc D into two discs, d and d', polarized in opposite directions ; the plane of d being: we will assume, vertical, and that of d' horizontal. The prism B will likewise give two images of the disc C ; the image c being polarized horizontally, and c' vertically. The size of the discs D C, and the separating power of the prism H are to be 80 arranged that the vertically polarized image d, and the hort zontally polarized image c, exactly overlap each other, forming, as shown in the figure, one compound disc c d, built up of ha the light from D and half that from C. (To be continued.) xoceedings of Sucieties. LIVERPOOL Amateur PIOTOGRAPHIC Association. The second out-door meeting was held on Monday, the 22nd ult., at Llangollen, North Wales. Notwithstanding that thio morning opened rather unfavourably, between twenty and thirl) * Comptei Rendug, April 25, 1853. t Proceedings of the British Association, Liverpool Meeting, 1854.
