THE CELLS. 



175 



of the third screw T likewise slid. The perforations MM', etc., correspond 

 to each other and to the associated wooden pegs mentioned above as 5. 



Figure 70 is an unconventional sketch of the cell when completely assem- 

 bled. 



A cell of the construction just described is very well suited to the study 

 of thin layers of solutions, in solvents of relatively high boiling-points, such 

 as water and amyl alcohol, but unless inclosed in some suitable vessel it 

 is not applicable to solvents of lower boiling-points, such as methyl alcohol, 

 ether, chloroform, acetone, etc. 



The cell used for making eye observations with the spectroscope had 

 to be constructed in such a way as to enable the observer to see simultane- 

 ously in the field of view of the telescope, the absorption spectra of two 

 solutions. This was necessitated by the very indefinite limits of absorption 

 possessed by most of the regions and bands of absorption of the substances 

 studied, i. e., salts of cobalt and copper. In other words, it would have 

 meant little or nothing to have attempted to observe one solution at a time, 

 by setting the vertical cross-hair at some definite wave-length, and to have 

 given the numerical 

 value of this wave- 

 length as the limit 

 of absorption or of 

 transmission, since 

 the gradation from 

 complete transmis- 

 sion to total extinc- 

 tion was, in general, so very gradual as to make the setting just proposed 

 practically impossible. Even if the cross-hairs had been set on a wave- 

 length for which it was estimated that the absorption was some definite 

 fraction of complete extinction, this very act of estimation would have varied 

 so greatly from one solution to the next as to cause the results to be 

 incomparable, and hence nearly worthless. 



When, on the contrary, two absorption spectra are symmetrically placed 

 in the field of view, it is easy to see at a glance all the qualitative differences 

 between the two spectra; and in addition, in many cases, to assign a number 

 of the right order of magnitude to the relative displacements of the regions 

 of absorption and of transmission in the superincumbent spectral bands. 



Obviously, the light incident upon one of the solutions had to be of equal 

 intensity to the light of the same wave-length incident upon the other solu- 

 tion. Two modes of procedure at least could lead to this result. One 

 scheme would be to use two equal, but entirely independent cells; to place 

 them symmetrically with respect to the source of light and the slit of the 

 collimator, and to send two beams of light, one through each cell, over equal 



FIG. 70. 



