APPARATUS 



217 



The degree of light absorption, and consequently the response in 

 the eye or in a photocell, is proportional to both the concentration 

 of the color substance and the length of the light path through the 

 solution. Therefore, a given quantity of color substance will effect 

 the same light absorption whether it is contained in a volume of 

 0.002 ml. and a 0.35 mm. light path is used (as in Richards' tech- 

 nique, page 195), or in a 0.060 ml. volume with a 10.5 mm. path. 

 Of course, the greatest absorption would be obtained by employing 

 the smallest volume with the longest light path. 



i 



Top % 



I 



t^ Light 



^ Penny" 



Light 

 Cuvette 



^ 



^ 



Diaphragm 

 (side) 



Diaphragm (Type A) 

 (face) 



Diaphragm (Type B) 



Fig. 77. Microcuvette and diaphragms. 

 Frovi Lowry and Bessey (1946) 



Lowry and Bessey Adaptation of Beckman Spectropho- 

 tometer to Measurements on Small Volumes. The special cu- 

 vettes used have the same 1 cm. light path as the macro variety, but 

 the width of the chamber has been reduced to 2 mm. or less (Fig. 

 77) . A 0.05 ml. volume of liquid will fill the cuvette to a height of 

 about 2.5 mm. The height of the cell is 25 mm. and its outside cross- 

 sectional dimensions are the same as those of the macro vessel. The 

 inner cross-sectional dimensions of the macrocuvette are 10 X 10 

 mm. Cuvettes having an internal measurement of 1 X 10 mm. have 

 also been used; they require 0.03 ml. liquid, but their use is more 

 difficult. 



