55 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



ing as if oue were to draw pen marks across a continuous spectrum at various 

 levels, sometimes apparently between the colors and sometimes in the midst of a 

 color. These dark lines are the so-called Fraunhofer Lines. Some of the princi- 

 pal ones have been lettered with Roman capitals, A. B. C. D. E. F. G. H. ( com- 

 mencing at the red end. The meaning of these lines was for a long time enig- 

 matical, but it is now known that they correspond with the bright lines of a line 

 spectrum (B). For example, if sodium is put in the flame of a spirit lamp it will 

 vaporize and become luminous. If this light is examined there will be seen one 

 or two bright yellow bands corresponding in position with D of the solar spectrum 

 (PI. V, Fig. 45). If now the spirit lamp-flame, colored by the incandescent so- 

 dium, is placed in the path of the electric light, and the light examined as before, 

 there will be a continuous spectrum, except dark lines in place of the bright 

 sodium lines. That is, the comparatively cool yellow light of the spirit lamp cuts 

 off or absorbs the intensely hot yellow light of the electric light ; and although 

 the spirit flame sends yellow light to the spectroscope it is so faint in comparison 

 with the electric light that the sodium lines appear dark. It is believed that in 

 the sun's atmosphere there are incandescent metal vapors (sodium, iron, etc.) 

 but that they are so cool in comparison with the rays of their wave length in the 

 sun that the cooler light of the incandescent metallic vapors absorbs the light of 

 corresponding wave length, and are, like the spirit lamp-flame, unable to make up 

 the loss, and therefore the presence of the dark lines. 



Absorption spectra from colored substances. — While the solar spectrum is an 

 absorption spectrum, the term is more commonly applied to the spectra obtained 

 with light which had passed through or has been reflected from colored objects 

 which are not self-luminous. 



It is the special purpose of the micro-spectroscope to investigate the spectra of 

 colored objects which are not self luminous, as blood and other liquids, various 

 minerals, as malazeit, etc. The spectra obtained by examining the light reflected 

 from these colored bodies or transmitted through them, possess, like the solar 

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

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

 stitution (PI. V, Fig. 45), 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. 



\ 130. Angstrom and Stokes Law of Absorption Spectra. — The wave lengths of 

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

 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 exam- 

 ined as a source of light its spectrum gives bright sodium lines, all the rest of the 

 spectrum being dark. 



\ 131. 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 



