n he source of light oest adapted i'or use with filters for 

 which all the wave lengths of the visible spectrum nre required 



^-w-vox-^ 



is the electric ?<rc. The advantages of fts use .- ; re, that all 



filters aan be u^ed in each exposure, insuring identical condition? 

 of temperature, constancy of illumination, etc. The disadvantages 



are several. In the first place, the intensity is constantly chang- 

 ing as the carbonslburn and the arc gets longer. r /orse, the lessen- 

 ing of the intensity nay not be the sane for all the wave lengths. 

 In the second place, fluctuations in the current cause large vari- 

 ations in the intensity. In tjje third place, unless an ifafefefa&J- 

 automatically adjusted arc is available, it is necessary to adjust 



the carbons by hand every five to fifteen minutes^, anfl when an 

 eight hour illumination is desired, tt$A entails considerable inconven- 

 ience. 



The dark boxes were 10x13 en. find 8 era. high, in one end ofe. 

 which a hole was cut so that one of the light filters, 6 x 6 cm. 

 might be fitted into it. The boxes were made light tight with 



tightly fitting covers, and were painted black inside to guard 



for uhe cultures 



against reflections within the box. The dishes used were made of 



r\ 



microscope slides cemented together with ainc eement so as to make 

 shallow oblong dishes 7.5x2.5 cm. and 1 cm. deep. It was deemed 

 necessary to use such flat sided dishes in order to prevent posrible 

 complications from reflected and refracted light in the 



