PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



prisms and lenses for the purpose of readily obtaining 

 spectra. The chief parts of it are a slit s, a prism p, 

 and a telescope L. Through the slit a narrow beam of 

 light is permitted to fall on the prism, which produces 

 the dispersion. A person looking through the tele- 

 scope sees an image of the spectrum. More than this 



Fig. 150. The Spectroscope. 



simple arrangement is used, however, in the construc- 

 tion of spectroscopes. In order that the rays coming 

 from the slit may be parallel a collimator is interposed 

 between the slit and the prism. This is a convex lens I. 

 It is fitted in a tube, at the outer end of which is the 

 slit (narrow, and cleanly cut, and placed vertically), 

 and it is distant its focal length from the slit, so that 

 the rays of light from the slit pass through the lens 

 and emerge parallel. The prism is placed with its 

 edge parallel to the slit, and receives the rays from 

 the collimator. Further, a convex lens may be placed 

 between the source of light and the slit to concentrate 

 the light on the slit, and thus obtain greater brilliancy. 

 The dispersed rays fall 011 the telescope, placed to 



