THE CELLS. 



173 



tilled water and extended as far as possible, no absorption, whether selective 

 or general, could be detected photographically. Therefore, the selective 

 absorption exhibited by the spectrograms was produced by the substance 

 or substances in solution. 



The cell just described was very well adapted to the study of plane par- 

 allel layers of liquid, having depths equal to and greater than one-half a 

 millimeter. On the contrary, its form did not lend itself as readily to the 

 photographic study of extremely thin layers of very opaque solutions, as did 

 that of another cell which had been designed by one of us for an earlier 

 investigation.* A description of the cell last named may not be super- 

 fluous in this place. This piece of apparatus was used in the present work, 

 in such a manner as to impart to the absorbing liquid the form of a wedge 

 or prism of zero thickness at the refracting edge. Consequently, the spec- 



n T 



A. 



FIG. 67. 



trograms obtained by this cell in the path of the beam of light, exhibit graph- 

 ically the variation which the limits of the regions of selective absorption 

 experience, when the depth of absorbing liquid varies linearly and continu- 

 ously from zero to a few tenths of a millimeter at the greatest thickness. 



The cell was made of five separable parts, as follows: (1) A brass frame- 

 work upon which the other parts rested; (2) a transparent tray without 

 a lid, which confined the liquid in proper bounds; (3) a transparent box- 

 like system which gave the upper surface of the liquid the desired position; 



(4) a vulcanized framework to hold the last-mentioned box in place, and 



(5) four mahogany pins or pegs to fasten the box to its framework. 



(1) A side view of this framework is presented in fig. 67. This projection 

 on a vertical plane is of natural size. There were three micrometer screws 

 all of the same pitch, viz, 1 turn=l/4S inch = 0.053 cm. The heads of 

 the screws were graduated on their upper surfaces in ten equal parts. The 

 screw T was in the immediate plane of the cell, while the remaining screws 

 (T r only is shown) were at the other end of the system, were equidistant 

 from this plane, and were as far apart as possible. The handle was denoted 

 by HH. A black fiducial mark F, on white ground, enabled the experi- 

 menter to tell what position the cell occupied with reference to the length 



*TJhler and Wood: Atlas of Absorption Spectra. 



