790 



HANDBOOK OF PHOTOGRAPHY 



meet the circumstances, due allowances being made in the interpretation of results 

 for the falling off in the qualitj' of the image. Similarlj^, to study the effects of various 

 wavelengths in the ultraviolet region on living cells and organisms, the slit image may 

 be altered and the effects observed visually in the searcher eyepiece or recorded 

 photographically. For such work the lower powers of the system should be used to 

 simplify the procedure. Perhaps the best practice is to photograph the field with a 

 wavelength of 2750 A., then irradiate it with some other wavelength, and again photo- 



MICRO- 

 SCOPE' 



SEARCHER 

 EYE PIECE 



o 



OBJECTIVE 



VERTICAL 

 ILLUMINATOR 



QUARTZ _^^A- - ' 



PLATE ^ \)- ^-~ — 



DIAPHRAGM f 



ADJUSTABLE ^URANIUM PRISM IRIS 



LENS GLASS SCREEN DIAPHRAGM 



m 



/' 



QUARTZ 

 PRISMS 



COLLI MATOR<Ctl> 



QUARTZ I 



SLIT ' 



■^\i/ 



SPARK 

 LECTRODES 

 Fig. 18. — Schematic diagram for optical system for working in the ultra\'iolet range when 



using reflected light. 



Carbon plate 

 variable rests fance'n 



Safety spark gap 

 \ Conc/ensers, 



Double pole snap 

 ire/ 



switch 



Step-up transformer I 

 Ph. 220 V. 60^ 1 



Sec. 10.000 V. 



77 or magnesiL 

 "k terminoils ■ 



Fig. 19. — Diagram showing electrical system for ultraviolet illumination. 



Cadmium or magnesium 

 spark terminals -'' 



graph it Mith 2750 A. This has the advantage of yielding good photographic images 

 but the disadvantage of having to refocus the substage condenser and illuminating sys- 

 tem for each change in wavelength. It must be remembered that, if the illumination is 

 not properly focused, the intensity of the ultraviolet light on the specimen per unit of 

 area is greatly reduced. When observing the lethal or stimulating effects of a given 

 wavelength, the results Mall mean little unless the illumination is properly focused and 

 centered. 



The optical system of the microscope as arranged for opaque objects is shown in 

 Fig. 18. The only difference in this assembly from that for transmitted light is the 

 elimination of the substage apparatus and the addition of vertical illumination. 



