THE BLOOD. 



249 



can be studied with the spectroscope, and which are so character- 

 istic as to serve for their identification. 



In principle a spectroscope consists of a prism which decomposes 

 the light from a narrow slit into a band of all the spectral colors. 

 A form of spectroscope in comm6n use is that shown in Fig. 105. 

 It consists of a tube, B, which has at one end a slit that can be 

 narrowed or widened by means of a screw. The light, having 

 passed through it, falls on an achromatic convex lens (called the 

 collimator) at the opposite end of the tube which renders the diver- 

 gent rays of light parallel. These parallel rays subsequently fall 

 on the prism, by which they are dispersed and directed into the 



FIG. 105. THE SPECTROSCOPE. A. Telescope. B. Tube for the admission of 

 light and carrying the collimator. C. Tube containing a scale, the image of 

 which when illuminated is reflected above the spectrum. D. The fluid exam- 

 ined. (Landois and Stirling.) 



tube, A, which is nothing more than a small telescope. On looking 

 into it at the ocular end the spectral colors are seen If the light 

 has been derived from the sun, the spectrum will present vertical dark 

 lines, the so-called Fraunhofer's lines. They are given from A to F 

 in Fig. 1 06. If a colored medium be held in front of the slit so that 

 the light has to pass through it first, certain dark bands will appear 

 in the spectrum, owing to the absorption of certain rays. 



Dilute solutions of arterial blood show two absorption bands 

 between the Fraunhofer lines, D and E, in the green and yellow 



