22G 



RADIATION HIOLOGY 



after the microscopic preparation. There are many possible types of 

 apparatus for this sort of procedure (see Caspersson, 1950; Loofl)()urow, 

 1950; Mellors et al., 1950; Blout et al., 1950). Since in the visible spec- 

 trum there are objectives and condensers, corrected both for chromatic 

 and spherical aberrations, the measurement of visible absorption spectra 

 is relatively simple, involving merely movement of the wave-length drum 

 of the monochromator. For ultraviolet absorption measurements, 

 Caspersson (193G) originally used the Zeiss-Kohler apparatus and meas- 

 ured absorption at each wave length by photography. This whole 

 instrument was designed for photography at one wave length, quite 



MAGNIFIER 



PHOTOTUBE 



POWER 

 SUPPLY 



Fig. 6-7. Diagram of a simplified apparatus for microphotometric study of cytological 

 preparations. {After Swift, 1950.) 



uncorrected for chromatic aberration, and its use for absorption curve 

 measurement by photography is extremely tedious. For each wave 

 length both condenser and objective must be refocused, and it is necessary 

 to carry out two measurements or make two photographs at each wave 

 length. This uncorrected optical system has been used for all the exten- 

 sive work of the Caspersson school, with many additions (Fig. 6-8), such 

 as an achromatic grating monochromator, extremely sensitive photore- 

 ceivers (measuring currents of the order of lO"'^ amp) and a polarizing 

 prism and special mechanical stages (Caspersson, 1950). The develop- 

 ment of achromatic reflecting objectives and condensers (Fig. 6-9) 

 (Brumberg, 1943; Burch, 1947; Grey, 1950; Norris et al, 1951; Barer, 

 1951) makes the problem of optical apparatus for ultraviolet absorption 

 measurements essentially as straightforward as in the visible spectrum. 

 With the instrument which has been developed by Sinsheimer a com- 



