No. 5.] 



STUDY OF ARC SPECTRA. 



371 



different conditions, and of the effect of the metallic spectra upon 

 the original carbon arc spectrum. 



Apparatus. 



The spectrum to be photographed was produced by means of 

 a Rowland concave grating, which was arranged in the usual way 

 upon a Brashear mounting. The latter consisted of a strong iron 

 frame, which carried two tracks, SG and SC, Fig. i, at right 

 angles to one another. At their point of intersection ^V, the slit 

 was placed. The carriage G, which contained the grating, moved 

 on one track, and the carriage C, for the camera box, on the other. 

 These two carriages were connected by an iron rod to which they 



M 



[ 



w 



-_> 



A- 



iOi 



W 



Fig. 1. 



were clamped in such a position as to keep the rod in the common 

 normal to the grating and photographic plate. A screw on the 

 camera box permitted the finer adjustments of position to be 

 made. 



The width of the slit was adjusted by means of a micrometer 

 screw, 0.1 mm. being the width ordinarily used. The length of 

 the slit was gauged by means of a diaphragm having a wedge- 

 shaped opening. A frame which held the slit could be turned by 

 means of a screw and spring so as to place the slit accurately par- 

 allel to the lines of the grating. 



The grating was of sLx-foot radius, and 14,000 lines to the 

 inch. 



Celluloid films were used, and, as these could be readily bent to 



