111.] 



USE OF PKISM AND LE 



approximately brought face to face with the improvement in- 

 troduced by Fraunhofer. This lens will throw an image of the 

 spectrum on a screen or a piece of paper placed at a distance 

 from it determined by the length of the focus of the lens. 



But Fraunhofer did more than this, he used a small telescope, 

 no screen was necessary, and the spectrum appeared to the 

 eye greatly magnified by the eyepiece. Similarly we may use 

 an opera-glass or a small telescope instead of the simple 

 spectacle lens. 



FIG. 9. Introduction of a lens to produce an image (c slit). 



While on this part of our subject we may as well say one 

 word on another addition to the prism and telescope as used by 

 Fraunhofer, made by Mr. Simnis in 1839. This has given us 

 the spectroscope as we now know it. 



Fraunhofer, as we have seen, in order to get good definition 

 with his little telescope, required the slit to be some considerable 

 distance (as much as 642 feet in one set of experiments) 

 in front of the prism; not only did this arrangement 

 require much space, but the various rays from the slit 

 entered the prism at different angles, and in consequence the 



