A CRITIQUE OF CYTOCHEMICAL METHODS 



227 



plete absorption curve can be run mechanically, without refocusing; 

 a density-wave length curve is recorded on a drum; and by a beam- 

 splitting mechanism and chopping the beams at two frec^uencies it is 

 possible to compensate for the transmission of the empty part of the slide 

 so that no second curve is necessary (Loofbourow, 1950). Many details 

 of techni(iue will be found in such references as Caspersson (1936, 1950); 



Fig. 6-8. Diagram of main instrument used in measuring ultraviolet absorption with 

 high accuracy and stability. A, mercury lamp; B, tungsten band lamp; C, mono- 

 chromator; D, exit slit of monochromator; E, lens; F, movable 90° quartz prism; G, 

 quartz plate, used with photocell to compensate for changes in the lamp; H, condenser; 

 /, object on slide; K, objective; L, ocular with adjustable diaphragm; il/, accurately 

 movable prism of fused quartz; N, rotating sector; 0, telescope for centering; P, 

 Kohler's rotating spark gap arrangement; R, photocell; *S, electrometer; T, leakage 

 resistance; U, four-step potentiometer; X, camera; F, Kohler focuser for the ultra- 

 violet, interchangeable with prism M. (After Caspersson, 1950.) 



Gersh and Baker (1943); Thorell (1947); Pollister and Ris (1947); Pol- 

 lister and Moses (1949); Swift (1950); and Pollister (1952c). 



An extreme simplification of the problem of instrumentation for 

 microspectrophotometry is to regard the whole apparatus as merely a 

 somewhat more complicated optical pathway than that in the conven- 

 tional devices which use absorption cuvettes, and to consider the micro- 

 scope as no more than an aid to locating an extremely small analytical 

 sample symmetrically in the optical pathwa}^ and delimiting the area to be 

 measured. If computations of concentrations and amounts are to be 



