112 THE ART OF PROJECTING. 



a lens in front of the orifice at about its focal length 

 distant from it, and placing the prism at its focus, and 

 inclined to the concentrated beam in the same way as 

 above. The spectrum will then be very bright and 

 some lines well marked. 



In order to show the Fraunhofer lines to advantage 

 it is necessary to have the room quite dark ; to use a 

 very narrow slit and a lens in conjunction with a good 

 triangular prism of flint glass or of bisulphide of 

 carbon. The diaphragm containing the slit through 

 which the light must pass should be placed close to the 

 opening in order to exclude all the light that is not 

 directly used. 



Fig. 89. 



This diaphragm may be made of pasteboard with 

 a slit cut in it three quarters of an inch long and 

 the fiftieth of an inch in width ; the edges should be 

 smooth and parallel. A lens with a focus of five or 

 six feet is best for sharp definition of the lines, but 

 one with a focus of only a foot or two may be used to 

 exhibit the large and more prominent of them. Place 

 the lens at such a distance from the slit as to project 

 it sharply upon the screen, at a distance from the lens, 

 say twenty feet. Then bring the triangular prism dose 



