222 PHOTOGRAPHING WITH THE MICROSCOPE [Cn. VII 



344. Difference of visual and actinic foci. Formerly there 

 was much difficulty experienced in photo-micrographing on account 

 of the difference in actinic and visual foci. Modern objectives are 

 less faulty in this respect and the apochromatics are practically free 

 from it. Since the introduction of orthochromatic or isochromatic 

 plates, and in many cases the use of color screens, but little trouble 

 has arisen from differences in the foci. This is especially true when 

 mono-chromatic light and even when petroleum light is used. In 

 case an objective has its visual and actinic foci at markedly different 

 levels it would be better to discard it for photography altogether, for 

 the estimation of the proper position of the sensitive plate after focus- 

 ing is only guesswork and the result is mere chance. If sharp pictures 

 cannot be obtained with an objective when petroleum light and ortho- 

 chromatic plates are used, the fault may not rest with the objective, 

 but with the plate holder and focusing screen. They should be very 

 carefully tested to see if there is coincidence in position of the focusing 

 screen and the sensitive film as described in 334. 



LIGHTING FOR PHOTO-MICROGRAPHY 



345. Light. The best light is sunlight. That has the defect 

 of not always being available, and of differing greatly in intensity 

 from hour to hour, day to day, and season to season. The sun does 

 not shine in the evening when many workers find the only opportunity 

 for work. Following the sunlight the electric light is the most intense 

 of the available lights. 



For many specimens daylight gives altogether the best results, 

 and as natural daylight is not constantly available the photo-microg- 

 rapher has now at his disposal the artificial daylight by the use of a 

 nitrogen-filled mazda lamp and daylight glass. The lantern for this 

 shown in fig. 129 was found to be excellent and the results obtained 

 by its use in photographing with powers up to the 1.5 mm. homogeneous 

 immersion were excellent. Of course any light filters which are 

 adapted to natural daylight would serve perfectly with the artificial 

 daylight. 



For all preparations needing a yellow color screen for daylight, a 

 petroleum or kerosene lamp gives good results for the majority of 



