EXPOSURES AND SPECTROGRAMS. 18i 



the capacity of the camera was limited to eight spectral bands, each 6 mm. 

 wide and spaced 0.05 mm. apart. 



Except when there were some very good reasons for doing otherwise, care 

 was taken to develop the halves of a film simultaneously, so as to subject 

 all of the exposures to exactly the same chemical conditions. When the 

 series of solutions comprised less than fifteen members, but more than eight, 

 the change from one half the film to the other was generally made as soon 

 as the number of solutions which had already been photographed, either 

 equaled the number of those that remained, or exceeded the latter number 

 by unity. The reason for so doing is doubtless obvious. For any one 

 solution the exposure with the Nernst glower was made first, the screen 

 Q of the spectrograph being, of course, vertical. Then this screen was set 

 horizontal, and the spark run for a definite length of time. Usually the 

 light from the glower was caused to pass through the absorbing liquid for 

 one and one-half minutes, and that from the spark for one and one-third 

 minutes. These two intervals of time were so related that when no 

 medium exhibiting appreciable absorption was in the path of the beams of 

 light, the photographic impressions successively produced by the radiations 

 from the Nernst glower and by the spark, blended into each other so well 

 that no discontinuity in the continuous background could be detected. The 

 lengths of the exposures will be given only when they differ from 90 seconds 

 and 80 seconds, respectively, for the glower and spark. The screen Q per- 

 formed the function of protecting the negative film from contamination from 

 the spectrum of the second order, which otherwise would have been super- 

 posed upon the spectrum of the first order. 



As soon as the light transmitted by one solution had been recorded photo- 

 graphically, another solution was substituted for the preceding one, and 

 also the film-holder was moved along by the rack and pinion system through 

 about 6.15mm.; 'that is, until a strip of unexposed film came into the field 

 of view of the grating. This strip was at a distance of a half-millimeter or so 

 from the region previously exposed. After all of the eight or less complete 

 photographic strips on one half-film had been exposed, the proper opening in 

 the slide screen or shutter L (see fig. 1) was brought into position and the 

 comparison spectrum impressed. 



When spectrograms were made with trichromatic plates and the cell 

 under consideration, the sequence of events was essentially the same as 

 that just explained, except in so far as the exposure of solutions to the light 

 from the spark were omitted. Further, since plates of this kind are not 

 as sensitive to orange and red light as the Seed films are to most of the 

 more refrangible colors, the time of exposure to the glower's radiations was 

 usually two minutes. 



Since the complete description of the manner of experimenting with the 

 wedge-shaped cell has already been given elsewhere, and especially since 



