160 .^HCRO-SPECTROSCOPE AND POLARISCOPE [ CH. VI 



216. Angstrom and Stokes' Law of Absorption Spectra The waves of 

 light absorbed by a body when light is transmitted through some of its sub- 

 stance are precisely the waves radiated from it when it becomes self-luminous. 

 For example, a piece of glass that is yellow when cool, gives out blue light 

 when it is hot enough to be self-luminous. Sodium vapor absorbs two bands 

 of yellow light (D lines); but when light is not sent through it, but itself is 

 luminous and examined as a source of light its spectrum gives bright sodium 

 lines, all the rest of the spectrum being dark (Fig. 136). 



2 217. Law of Color. The light reaching the eye from a colored, solid, 

 liquid or gaseous body lighted with white light, will be that due to white light 

 less the light waves that have been absorbed by the colored body. Or in other 

 words, it will be due to the wave lengths of light that finally reach the eye 

 from the object. For example, a thin layer of blood under the microscope 

 will appear yellowish green, but a thick layer will appear pure red. If now 

 these two layers are examined with a micro-spectroscope, the thin layer will 

 show all the colors, but the red end will be slightly, and the blue end consid- 

 erably restricted, and some of the colors will appear considerably lessened in 

 intensity. Finally there may appear two shadow-like bands, or if the layer is 

 thick enough, two well-defined dark bands in the green ($ 232). 



If the thick layer is examined in the same way, the spectrum will show 

 only red with a little orange light, all the rest being absorbed. Thus the 

 spectroscope shows which colors remain, in part or wholly, and it is the mix- 

 ture of this remaining or unabsorbed light that gives color to the object. 



\ 218. Complementary Spectra. While it is believed that Angstrom's 

 law {\ 216) is correct, there are many bodies on which it cannot be tested, as 

 they change in chemical or molecular constitution before reaching a suffi- 

 ciently high temperature to become luminous. There are compounds, how- 

 ever, like those of didymium, erbium and terbium, which do not change with 

 the heat necessary to render them luminous, and with them the incandescence 

 and absorption spectra are mutually complementary, the one presenting bright 

 lines where the other presents dark ones (Daniell). 



ADJUSTING THE MICRO-SPECTROSCOPE 



219. The micro-spectroscope, or spectroscopic ocular, is put 

 in the place of the ordinary ocular in the microscope, and clamped 

 to the top of the tube by means of a screw for the purpose. 



$ 220. Adjustment of the Slit. In place of the ordinary 

 diaphragm with circular opening, the spectral ocular has a dia- 

 phragm composed of two movable knife edges by which a slit-like 

 opening of greater or less width and length may be obtained at will 

 by the use of screws for the purpose. To adjust the slit, depress 

 the lever holding the prism-tube in position over the ocular, and 



