I 6 ATLAS OF ABSORPTION SPECTRA. 



plates shows that mechanical shifts, in wave-lengths, of the strips on one 

 complete spectrogram, with reference to one another, exist. This may mar 

 the appearance of the photographs somewhat, but the ultra-violet spark lines 

 show the magnitude of the displacements so that corrections can be made, 

 and hence the ultimate scientific value of the results is not decreased. 



The order of events in taking a complete negative of tJiree strips was 

 invariably as follows : The thickest layer of absorbing liquid was over the 

 opening of the slit first, then the intermediate layer, and last of all, the 

 thinnest layer, which usually tapered to infinitesimal depth. This sequence 

 enabled the comparison spectrum to be taken by moving the shutter, L, 

 without jarring the film-holder, so as to minimize the shift of this spectrum 

 relative to the adjacent photographic strip. For negatives of more than three 

 strips precisely the reverse succession was adopted because it was easier to 

 commence with the cell in adjustment and then to raise the quartz wedge 

 parallel to itself than to lower all three micrometer screws by the same 

 number of turns until the quartz wedge just barely came into contact with 

 the quartz bottom of the cell. With the screen Q horizontal the first expo- 

 sure with the spark was taken. The screen was lowered and the second 

 exposure was made, this time with the Nernst glower. These two exposures 

 produced the first of the three photographic strips. Next the film-holder 

 and cell were moved the proper distances, as explained above. The glower 

 and spark exposures followed in the order named. After again moving the 

 film-holder and cell, the fifth and sixth exposures were produced by 

 the spark and glower respectively. Finally the cell and diaphragm were 

 removed from the slit, another opening in the shutter was adjusted before 

 the film, and the comparison spectrum impressed. In general, the glower 

 exposures lasted 60 seconds, the ultra-violet exposures 75 seconds, and the 

 comparison exposures 35 seconds. The width of the slit was always 0.008 

 cm. In any one complete spectrogram the exposures to the Nernst light 

 were all equal to each other and those for the ultra-violet were related to 

 one another in the same manner. Experience showed that the intervals 60 

 and 75 seconds were best suited to cause the overlapping ends of the photo- 

 graphic impressions to blend as if they had been produced simultaneously 

 by light from a single source. With the longest exposures used, the light 

 from the glower did not affect the films and plates for wave-lengths as short 

 as 0.315// and, since the field photographed did not comprise wave-lengths 

 longer than 0.63,^, there was no trouble produced bj' the ultra-violet of the 

 second order. The screen Q took care of this matter so far as the spark 

 exposures were concerned. Figures 14 and 15, plate 4, indicate how the 

 processes just explained can be extended to negatives as wide as may be 

 desirable and hence to as deep la3'ers of absorbing liquid as may be wished.* 



*0f course, a cell deeper than 6 ram. would be necessary if the matter were pushed very far. 



