LIGHT 601 glass trough with parallel sides) in front of the narrow slit through which the light passes, and in such a position that one half of the slit only is thus covered. We have then side by side, under precisely similar circumstances, two spectra to be compared (one altered by absorption, the other not) ; and very minute differences between them can thus be detected. When the medium produces a general weakening of the whole spectrum, as well as particular local absorptions, the white light passing through the other half of the slit may be weakened to any desired extent by re flexion at the proper incidence from a plate of glass, before it falls on the slit. To give a satisfactory representation of the phenomena of absorption spectra by the help of a woodcut is not easy. The highest artistic skill could not adequately represent the ordinary solar spectrum by the use of the finest pigments. All optical colour phenomena must be seen, they cannot be reproduced by painting. In such circumstances the simplest method of indicating the locality and amount of the absorp tion is the best. As we have already seen that we cannot by the eye judge of the relative intensities of lights which differ much in colour, we shall represent the normal spectrum (for our present purpose) as equally bright throughout, and indicate the absorption at different parts by shading of various degrees of depth. A few of the Fraunhofer lines are introduced to indicate (in the absence of colour) the parts of the spectrum which are attacked by the various absorbents. These lines are, of course, in the same abso lute positions in all the various spectra ; for the spectra are all supposed to be given by the same prism. The line B is in the red, D in the orange, E and F in the green, and Cr in the indigo. They correspond, as we have already said, to perfectly definite kinds of homogeneous light, and therefore adequately represent the distribution of colours in the spectrum, however much irrationality of dispersion may bo shown by the material of the prism. In fig. 26 a represents the spectrum of light which has passed through diluted blood ; /3 shows the spectrum when the blood has been acted on by a reducing agent ; and y the spectrum when the blood has been altered by acidu- lation with acetic or tartaric acid. These figures are taken from an important paper by Stokes (Proceedings of the Royal Society, 1864). Fig. 27 shows in a rude way the absorption by cobalt glass cut in wedge form, and corrected by an equal prism of clear glass. The commonly received method of calculating the ab sorption by layers of gradually increasing thickness is to suppose that, if a layer of unit thickness weakens in any ratio the intensity of any particular homogeneous ray, another unit layer will farther weaken in the same ratio that which reaches it, and so on. Thus the amount which passesthrough a number of layers diminishes in geometrical progression, while the number of layers increases in arithmetical pro gression. This is certainly true (neglecting the amount reflected), unless the intensity of the light have an effect on the percentage transmitted. And fig. 27 shows, in a very striking manner, the difference between similar terms of different geometric series as the common ratio becomes less and less. This ratio is not much less than 1 for cer tain red and blue rays, is smaller for yellow, and is very small for the rest of the red, for orange, and for green. The latter colours are therefore rapidly got rid of with in creasing thickness ; then the yellow becomes too feeble to be seen ; while, even after the blue becomes almost insen sible, the specially favoured red rays are still transmitted in sufficient quantity to be observed. If r be the fraction of any species of homogeneous liglit vliicli is transmitted by a layer of unit thickness, that transmitted by a layer of thickness x is r*. The following little table will greatly assist the reader in understanding the relative rapidity of extinction of different rays passing through various thicknesses of an absorb ing medium. It is a table of double entry, the first column giving various values of x, and the upper row various values of r, while the value of r* is in the same column as that of r and in the same row as that of x. 1 1 0-99 0-9 0-5 o-i o 1 0-98 0-81 0-25 o-oi 5 1 0-951 0-59 0-03 O OOOOl 10 1 0-904 0-349 0-0009 100 1 0-3C6 0-00003 Thus a ray which loses 1 per eent. in unit thickness still preserves more than 90 per cent, after passing through ten units. But a ray which loses 10 per cent, in the first unit (and which, therefore, will thus far appear scarcely more weakened than the first) is reduced to 35 per cent, by passage through ten units. After passing through a hundred units the first ray has lost only 63 per cent., the second is practically invisible. In thin plates cobalt glass is blue, because the particular red which it does not absorb freely forms only a small fraction of the whole transmitted rays ; while in thick masses it is nearly red, for then little but this favoured red is transmitted. For a similar reason Condy s fluid (per manganate of potash) changes its tint in a very singular manner (even when preserved from the action of the air) by gradual dilution with water. The imperfection of the achromatism of the eye is readily Defect of proved by looking through a plate of cobalt glass at a small achrom- hole in the window-shutter of a dark room. The hole at first j tlsm iu appears red with a blue space round it ; but, by an effort of the muscles of the eye, we can see the hole blue, and then there is a red space surrounding it. Rays of so widely different refractive index cannot be seen in focus simultaneously. Very curious effects are produced when we examine a landscape through such a glass. Foliage of certain kinds scatters scarcely any blue rays, and therefore appears reddish. Bluish-greens, again, which scatter very little red, appear blue. The effects may be exaggerated in a very striking degree by combining the absorptions of two or more media, so as to allow of the free transmission of a few, far detached, portions of the spectrum. Brewster made the very singular discovery that a solu tion of oxalate of chromium and potash produces one solitary, narrow, absorption band, almost resembling one of the broader lines in the solar spectrum. Closely connected with intense local absorption in certain Abnor- parts of the spectrum is the phenomenon of abnormal m . al dispersion, one of the most singular discoveries of modern g^ er times. It seems to have been first observed by Fox XIV. 76 *
