CH. XV] PHOTOGRAPHY OF SPECTRA 643 



USE IN PHOTOGRAPHY 



908. Apparatus. Slit, prism, grating, symmetrical photo- 

 graphic objective, camera bellows, bromide paper, photographic 

 plate. 



The systems described above for the demonstration of spectra on 

 a small scale (fig. 368, 371), may be employed for the photography 

 of spectra. Such a system can be used, for example, to determine 

 the wave-lengths of the radiation to which bromide paper is 

 sensitive. If the bromide paper is held firmly against a rigid sup- 

 port so that the spectrum of a right angle arc may fall upon it, the 



FIG. 371. ARRANGEMENT FOR THE PHO- 

 TOGRAPHY OF SPECTRA. 



S Slit. 



O l Front combination of a photographic 

 objective. 



0,, Back combination of the objective. 



P Prism between the two combinations 

 of the photographic objective. 



R V The spectrum in focus on the photographic 

 plate. 



Plate The sensitive photographic plate in the 

 camera. 



paper will be found to be blackened where blue and ultra-violet 

 light struck it, but the red and green will show no action at all. 

 If dry plates, however, are used a more complete system of shield- 

 ing from the light will be required. The second lens O 2 , may be 

 held in a camera box as shown in figure 371. A more elaborate 

 system for making several exposures on the same plate is not here 

 described because, while good results may be obtained with such 

 apparatus by sufficient labor, it is more satisfactory to use one of 

 the regular spectrographic cameras. 



