284 



THE BLOOD. 



orange, yellow, green, blue, and indigo ; but if it is the sun which 

 is the source of light, though this is an incandescent body, still its 

 light passes through an atmosphere which absorbs certain portions 

 of the light, and the spectrum is, therefore, crossed by dark lines, 

 Fraunho/er lines. These lines are fixed, and the more distinct 

 ones are designated by letters of the alphabet ; thus in the red are 

 A, B, and C ; in the yellow D, etc. As the atmosphere of the 

 sun absorbs certain parts of the light which is transmitted through 

 it, so do many transparent substances, and the spectra of such 

 substances are known as absorbent spectra, in contradistinction to 

 continuous spectra, in which no lines or bands appear. If, there- 

 fore, the light of the sun or that from any other source of illumi- 



FIG. 153. The hematinometer. 



FIG. 154. The hematoscope. 



nation is passed through a solution of one of these substances, in 

 its spectrum dark bands will be seen at definite places and in 

 definite numbers, and the identity of such substances can thus be 

 determined. The positions occupied by these bands may be 

 designated either by stating their relation "to the Fraunhofer lines, 

 or the wave-length of the portions of the spectrum between which 

 absorption takes place, this being determined by a scale which is 

 provided for this purpose. 



For the description of the spectra of hemoglobin and its- 

 derivatives, and the use of the spectroscope in their differentiation, 

 we are especially indebted to the section on " Hemoglobin," by 

 Gamgee, in Schafer's Physiology. 



