138 



MICRO-SPECTROSCOPE AND POLARISCOPE \_CH. VI 



spectrum dark lines or bands, but the bands are usually much wider and less 

 sharply defined. Their number and position depend on the substance or its con- 

 stitution (Fig. 122), and their width, in part, upon the thickness of the body. 

 With some colored bodies, no definite bands are present. The spectrum is simply 

 restricted at one or both ends and various of the other colors are considerably 

 lessened in intensity. This is true of many colored fruits. 



\ 194. Angstrom and Stokes' Law of Absorption Spectra. The waves of 

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



FIG. 122. Absorption spectrum of Oxy-hemoglobin or arterial blood (/) and 

 of Ho moglobin or venous blood (2). (From Gamgee and McMunn}. 



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

 spectrum (\ 192}. 



.90, .80, .70, .60, .'jo, .40. Wave lengths in microns, as shown in Angstrom's 

 scale ( 202). It will be seen that the wave lengths increase toward the red and 

 decrease toward the violet end of the spectrmn. 



Red, Yellow, Orange, 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 

 spectrum being dark (Fig. 121). 



195. 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 ob- 

 ject. 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 considerably restricted, and some of 

 the colors will appear considerably lessened in intensity. Finally there may ap- 

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

 dark bands in the green (g 210). 



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 remain- 

 ing or unabsorbed light that gives color to the object. 



