628 PROJECTION OF SPECTRA [Cn. XV 



886. Source of light. In order to demonstrate an absorption 

 spectrum of a substance it is necessary to use a source of light 

 which has a continuous spectrum in order not to confuse the dis- 

 continuities of the light source with those caused by the absorbing 

 medium. 



887. Sunlight. Sunlight while not having a perfectly con- 

 tinuous spectrum is near enough to it for most purposes. The im- 

 age of the sun should be focused on the slit as described for the 

 crater of the arc lamp ( 376, 833-834). 



888. Carbon arc lamp. In the visible parts of the spectrum 

 the carbon arc lamp will give a continuous spectrum, using pref- 

 erably the positive crater of the direct current arc, although alter- 

 nating current will answer. 



889. The uranium arc. Using the vertical arc, the lower 

 electrode made of uranium oxide is connected to the positive 

 terminal as described in 905. About 4 to 6 amperes should 

 be used and a fairly continuous spectrum will be obtained. This 

 continuous spectrum will extend well into the ultra-violet and can 

 be observed by using an anthracene screen ( 899) . 



890. The Nernst lamp. This can be used if necessary. 

 Focus one filament on the slit or use a very long focus lens (one of 

 100 cm. focus or longer) and use the filament as a line source of 

 light. The spectrum is continuous but not very bright. 



The tungsten incandescent lamp. One filament is focused on 

 the screen by the objective and a slit is placed over the side of the 

 bulb so that none of the other filaments show on the screen. This 

 single filament acts as a line source of light. 



OPTICAL SYSTEM 



891. The optical system for the projection of spectra is 

 shown in fig. 353. First a condenser C, is put in front of the arc. 

 This condenser brings an image of the arc to a focus at the slit. 

 An objective is put at O, so as to focus an image of the slit 

 on a distant screen at i. When a prism is used to disperse 

 the light into a spectrum, it is placed at P and turned as shown. 



