DUBOSCQ COLORIMETER 



153 



light; they contain the solutions to be tested. Two solid cylindrical 

 plungers of optical glass, matched for color and with optically plane 

 and parallel ends, are lowered to various depths into the cups. These 

 plungers can be set independently so that various depths of liquid may 

 be examined. The two beams of light which pass through the glass 



ii 



in'-*" -=3=; 



(&) 



Fig. IV-13. (a) Path of light rays through a Duboscq colorimeter, (b) Path of 

 light rays through a hemoglobinometer. (By courtesy of Bausch and Lomb Opti- 

 cal Company.) 



plungers are brought to a common axis by means of the rhombohedral 

 prisms D. The biprism refracting system C places the two images side 

 by side so that the light from each cup illuminates half the field. The 

 eyepiece BA by which the observer sees both fields with one eye is 

 focused on the line of separation of the two fields. 



The depth of the two columns of liquid may thus be altered by moving 

 the plungers independently in their respective cups until the two halves 

 of the field are identical in brightness. When these conditions are ful- 

 filled, the concentrations of the two solutions are inversely proportional 

 to the depths, which are read on the scales of the instrument. Such a 

 subjective setting is good to 1 per cent. 



For best results the source must not vary in intensity or color-tempera- 

 ture. Artificial illumination is therefore recommended. 



To increase the sensitivity of color match in working with a blue 

 field, a yellow filter is added over the eye lens, so as to work with a 

 neutral green. Under these circumstances, should the layer of the 



