636 



PROJECTION OF SPECTRA 



[CH. XV 



the paper with a brush. This screen will show a brilliant green 

 fluorescence wherever ultra-violet light strikes it. When the 

 spectrum of an arc is projected upon such a screen, not only is the 

 visible red, green, and blue to be seen, but also beyond the blue end 

 is a vivid green fluorescence which indicates the presence of ultra- 

 violet light. 



FIG. 364. ILLUMINATION OF THE SLIT FOR THE PROJECTION OF SPECTRA. 



A The image of the arc focused to a small spot on the slit. The objective 

 is filled with light. The spectrum will be bright but a mere line. 



B The image of the arc slightly out of focus giving a higher spectrum but 

 not so bright. 



C The slit next to the condenser in the lantern-slide position. This gives 

 a relatively dim spectrum and illuminates a greater height of slit than is used 

 for the spectrum. 



900. Illumination of the slit. Excellent results may be 

 obtained by focusing the arc on the slit by the condenser. This 

 gives an intense illumination but the spectrum is not very high, in 

 fact, it may be a mere line of color. To remedy this, the slit may 

 be brought closer to the condenser than the crater image. Thit 

 increases the height of the spectrum but reduces its intensity. Is 



