12 



STUDY OF ABSORPTION SPECTRA 



plates were sufficiently piano-parallel was determined by the character 

 of the interference fringes which they gave. These glass plates closing 

 the ends of the cells were 1 mm. thick. We found some difficulty in 

 procuring plates of the best optical glass which were piano-parallel. 

 Many times when the plates were sufficiently piano-parallel for our 

 purpose, the Wood's metal surrounding the edges of the plates would, 

 on cooling or setting, so warp the plate that it was necessary to reset it. 

 This often had to be repeated several times to secure the desired result. 



The distance between the glass ends was regulated by means of a 

 finely threaded nut which screwed on to the outer cylinder. Each 

 complete revolution of this nut raised or lowered the inner cylinder 

 just 1 mm. The nut was calibrated in 100 divisions by means of a 

 dividing engine, and with this nut we could readily and accurately 

 adjust the distance between the glass ends to less than 0.01 mm. 



The brass cells were first heavily plated with silver, being taken out 

 of the plating bath from time to time and thoroughly rubbed with the 

 finest crocus powder. A heavy gold plate was then deposited on this 

 silver surface. Unless the gold was of sufficient thickness to cover 

 well all of the exposed portions of the Wood's metal, it was found that 

 the concentrated and strongly hydrolyzed solutions which we studied 

 would act upon the Wood's metal, giving rise to streamers in the 

 solutions and often to an opalescence. This would, of course, lead to 

 considerable error. The solutions used in making the measurements 

 were always perfectly clear, and if any opalescence developed in them 

 while making the measurements, the solutions were discarded and the 

 cells were at once replated. 



TABLE 1. Testing the two cells. 



The two cells were tested from time to time to see if, under com- 

 parable conditions, they both gave the same deflections. This was 

 usually done by placing 10 mm. of water in each cell and noting the 

 actual deflections as given by the radiomicrometer for the various 

 wave-lengths of light. Table 1 contains the results for one such test. 

 The given wave-lengths of light would be obtained by turning the 

 drum-head of the spectrometer at intervals of ten divisions over the 

 entire region of the spectrum which we studied. In all tests the 

 deflections for the two cells agreed as closely as those given in table 1. 



