CH. XV] 



PROJECTION OF SPECTRA 



635 



will show the visible parts of the spectrum very well, that is, it 

 will show the red, green, and blue parts. 



In order to illustrate the ultra-violet parts of the spectrum, use 

 a screen coated with anthracene. A suitable screen can be made 



FIG. 363. USE OF A CONCAVE REFLECTING GRATING FOR THE PROJECTION 



OF SPECTRA. 

 S Slit. 



Objective. 



G Concave reflecting grating. 



1 V R Image of the slit (i) and of the spectrum (R V). 



Note that in order to get a sharp image of the slit at i, it is necessary to have 

 diverging light strike the grating. The spectrum R V will be sharply denned 

 under these conditions. 



by taking a piece of white cardboard 50 cm. x 75 cm. and painting it 

 with a suspension of anthracene in xylene. The anthracene used 

 is the ordinary commercial variety of resublimed anthracene. 

 Only enough xylene is used to make the mixture so it can be put on 



