SELECTION OF MATERIAL, APPARATUS, ETC. 9 



the bottom of the cell and (because it was desirable to reduce the number of 

 reflecting surfaces to a minimum) a vertical t\^pe of spectrograph. 



The cell comprised live separable parts, as follows: (i) 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 boxlike 

 S3'stem which gave the upper surface of the liquid the desired position; (4) a 

 vulcanite framework to hold the last mentioned box in place ; and (5) four 

 mahogan)' pins or pegs to fasten the box to its framework. 



(i) A side view of this framework is presented in figure 2. There 

 were three micrometer screws, all of the same pitch, viz : i turn =^ k in. 



^0.053 cm. The heads 

 of the screws were grad- 

 uated, on their upper sur- 

 faces, in ten equal parts. 

 Fig. 2.-Four-fifths natural size. The scrcw T was in the 



medial plane of the cell while the remaining screws (T' only is shown) were at 

 the other end of the system, were equidistant from this plane, and were as far 

 apart as possible. The micrometer screws called for vertical scales on the 

 adjacent brass-work to count whole turns. The handle is denoted by HH. 

 A black fiducial mark, F, on a white ground, enabled the experimenter to 

 tell what position the cell occupied with reference to the length of the slit of 

 the spectrograph. The lower end of F moved over a scale parallel to the 

 slit and in the plane of the jaws of the latter. The flange at the bottom of 

 the framework was made of brass only 0.014 cm. thick so that the absorb- 

 ing medium might be as near the slit as possible. 



(2) An accurately ground, plane-parallel plate of quartz 40 mm. long, 

 18.5 mm. ^yide, and 2 mm. thick had cemented to its periphery four 

 rectangular sheets of thin glass 8 mm. high. Hence, the greatest depth of 

 liquid which could be studied by the aid of this cell was 6 mm. 



(3) In figure 3, a, 6, c, and d designate the vertices 

 of the section of a quartz plate, made by a plane per- 

 pendicular to the plane the trace of which is the line 



M M 



O O 



ad. ad was 2 mm., ad wa.s 34.8 mm., and the angle Fig. 3. Four-fifihs nat. size, 

 between the planes of ad and dc was 55 minutes of arc. The horizontal 

 width of the wedge was 10 mm. Glass walls surrounded three sides of the 

 wedge, as the outline indicates. The reason for using the quartz wedge was 

 to counteract the deviation and dispersion produced by the solution in the 

 cell. The angle of the liquid wedge could be varied until the deviation 

 effected by the quartz wedge nullified the average action of the absorbing 

 solution. At first it was supposed that with liquid wedges of 15 or so 

 minutes of arc a plane-parallel quartz plate could be used successfully instead 



