Ch. VIII] ARRANGING THE SPECTROSCOPE 253 



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

 lines, all the rest of the spectrum being dark (fig. 148). 



§ 398. Law of color. — The light reaching the eye from a colored 

 solid, liquid, or gaseous body hghted with white hght 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 hght 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 

 colors, but the red end will be shghtly, and the blue end considerably, 

 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 



(§413)- . 



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



show only red with a httle orange hght, all the rest being absorbed. 

 Thus the spectroscope shows which colors remain, in part or wholly, 

 and it is the mixture of this remaining or unabsorbed light that gives 

 color to the object. 



§ 399. Complementary spectra. — While it is beUeved that 

 Angstrom's law (§ 398) is correct, there are many bodies on which it 

 cannot be tested, as they change in chemical or molecular constitu- 

 tion before reaching a suflSciently high temperature to become lumi- 

 nous. There are compounds, however, like those of didymium, 

 erbium, and terbium, which do not change with the heat necessary to 

 render them Imninous, and with them the incandescent and ab- 

 sorption spectra are mutually complementary, the one presenting 

 bright lines where the other presents dark ones (t)aniell). 



Adjctsting the Micro-spectroscope 

 § 400. 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 side screw for the purpose. 



§ 401. Adjustment of the slit. — In place of the ordinary dia- 

 phragm with circular opening, the spectral ocular has a diaphragm 



