124 



MICRO-SPECTROSCOPE AND POLAR/SCOPE. \_CH. VI. 



.90 30 



Fig. ii6. Absorption Spectrum of Oxy-Hemogtobin or arterial blood (/) and of 

 Hemoglobin or venous blood (2). (From Gamgee and McMunn.) 



A, B, C, D, E, F, G, H. Some of the Principal Fraunhofer lines of the solar 

 spectrum (\ jSj). 



.go, .80, .70, .60, .50, .40. Wave lengths in microns, as shown in Angstrom's 

 scale (\ 193). It will be seen that the wave lengths increase toward the red and 

 decrease toward the violet end of the spectrum. 



Red, Orange, Yellow, etc. Color regions of the spectrum. Indigo should come 

 between the blue and the violet to complete the seven colors usually given. It was 

 omitted through inadvertence. 



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

 ample, 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 spec- 

 trum being dark. 



$ 186. 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 exam- 

 ined with a micro-spectroscope, the thin layer will show all the colors, but the red 

 end will be slightly, and the blue end considerably restricted, and some of the colors 

 will appear of considerably lessened intensity. Finally there may appear two 

 shadow-like bands, or if the layer is thick enough, two well- defined dark bands in 

 the green (§ 202). 



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 mixture of this remaining or 

 unabsorbed light that gives color to the object. 



\ 187. Complementary Spectra.— While it is believed that Angstrom's law (? 185) 

 is correct, there are many bodies on which it cannot be tested, as they change in 

 chemical or molecular constitution before reaching a sufficiently high temperature 

 to become luminous. There are compounds, however, 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). 



