LIGHT. 113 



to the lens as shown : The light will be deflected and 

 dispersed, and the screen should now be brought 

 where the spectrum will fall perpendicularly upon it, 

 and at the same distance from the lens that it was 

 before, namely, twenty feet. Turn the prism until the 

 spectrum has its least deviation, which will be found by 

 a little trial. The Fraunhofer lines should appear. If 

 they are indistinct, move both the lens and prism back 

 or forward in the beam until they are distinct, for it is 

 now only a matter of focussing. 



If the lens has a focus five or six feet distant it will 

 need to be quite as far from the slit as the length of 

 its focus, and the screen adjusted as before, but the 

 lines should appear plainer and in greater number. 

 With such a lens and a good glass prism the spectrum 

 should be about five feet long, and with good focussing 

 the D line should be seen double. These lines may 

 be seen by a large number by moving the screen edge- 

 wise an inch or two. 



One may use a condenser and converge a large beam 

 upon the slit. This will make the spectrum brighter 

 and permit a narrower slit to be used, but the definition 

 of the lines is not so good as when parallel rays fall 

 upon the lens. If the object be to project a spectrum 

 that shall be well defined upon its sides and to show 

 only the more prominent lines, let the slit be made as 

 broad as the twentieth of an inch ; a lens with about a 

 foot focus may be used to project the slit in the ordi- 

 nary way, and the prism placed at the focus and turned 

 to its angle of least deviation, which, as before, must be 

 found by trial. In this way a beautiful and well-defined 

 spectrum will be produced, which at the distance of 

 twenty feet would be about five feet long and two feet 

 broad. 



8 



