14 ATLAS OF ABSORPTION SPECTRA. 



MANNER OF EXPERIMENTING. 



SOLUTIONS. 



A small, known mass of a selected dye was carefully weighed on a 

 chemical balance, and put at the bottom of a medium-sized test-tube. 

 Then distilled water was run from a burette into the test-tube, and the 

 latter shaken up from time to time, until the resulting solution appeared to 

 have the proper concentration. As would be expected, practice produced 

 skill in judging absorption of visible light, but to get the right concentration 

 with respect to ultra-violet light was not always so eas}'. The greatest error 

 in measuring the solvents was about 0.2 per cent. Since the concentrations 

 are only intended to serve as general guides to an understanding of the 

 spectrograms, a higher degree of accuracy would have been superfluous. 

 Neither was there any reason, in general, for noting the volume of solutio7t 

 which contained a known number of grams of pure solvend; in other words, 

 changes in volume due to the processes of solution were not regarded. 



ADJUSTMENT OF THE CELL. 



Especial care was taken to remove all coloring matter from the cell 

 before introducing another solution into it. Dust caused more trouble than 

 anything else. After cleaning the quartz and glass elements of the cell the 

 various parts of the latter were assembled and, when a prism of liquid was 

 to be studied, the micrometer screws regulated in the following manner : 

 All the screws were turned down so as not to touch the vulcanite framework, 

 and thus to cause the quartz wedge to rest on the quartz plate. Then the 

 screw T had its point elevated again and again until it just touched the 

 deepest part of the depression P. {^Sce figures 2, 3, 4, and 5.) This 

 condition was attained by gently rocking the S3fstem around the edge d of 

 the quartz wedge, somewhat after the fashion of experimenting with certain 

 types of spherometer. Thus the zero position of the cell was determined, 

 before each experiment, of course. Next, guided by the circular and plane 

 scales, the observer turned up the screw T until the desired angle, between 

 the wedge and plate, was known to obtain. After this, the screw corre- 

 sponding to T' was turned up until its tip projected far enough into the groove 

 P" to prevent the quartz wedge and its accessories from sliding over the 

 quartz plate around the point T as pivot, but yet not far enough to raise the 

 vulcanite frame the least bit. Finally, a small amount of the solution was 

 poured into the cell and the latter was then placed on the very thin brass 

 sheet which rested upon and protected the jaws of the slit. 



As soon as the cell was placed over the slit and the glower had been 

 lighted the cell was moved forward and backward, parallel to the slit, while 

 one edge of the field of view was examined with an ej'e-piece, until a position 

 of the cell was obtained for which the light passing through the quartz wedge 



