462 A. C. BANFIELD OX A METHOD OF PREPARING 



stereoscopic size, 6| X 3| inches. The formula I have mentioned: 

 with regard to the objective separation resolves itself in practice 

 into two parallel lines drawn on the focusing-screen 62 mm., 

 apart, by means of the stage. The object is moved till one of the 

 lines cuts the image centrally, and the first exposure made ; it is 

 then transferred to the other line, when a second exposure will 

 give us our truly stereoscopic pair. 



The lenses which I prefer for the low-power work which appeals- 

 peculiar ly to me are the wonderful little photo-objectives sold by 

 Zeiss under the name of the " Planars." They are perfectly 

 corrected, and, as their name implies, their field is very flat — a 

 very great advantage when the object is displaced from one 

 side to the other of the field as I have explained. Their only 

 disadvantage is that their aperture is so low that they cannot 

 be used for high magnifications ; when used at the full aperture 

 of F 4, the limit is about 72 diameters, at which diffraction effects 

 appear. The two which I have are of 20 and 35 mm. focus. I 

 have also an objective of a similar type, of 42 mm. focus, made by 

 Leitz, the performance of which is in every way admirable. It 

 is very useful for such comparatively large objects as Mycetozoa, 

 Foraminifera, etc. 



With regard to the illuminating of the object I much prefer 

 incident light. All the stereo-micrographs I have seen of objects 

 taken by transmitted light seem to me to be very unsatisfactory,, 

 through diffraction and shadow effects. An object to photograph 

 well from a stereoscopic point of view should be full of detail 

 in every part. I believe that some very beautiful high-power 

 photographs could be made of diatoms or similar objects by using 

 a vertical illuminator. The exposures would be very long, but I 

 live in hopes of trying something in this direction before long. 



With regard to an illuminant, I have found the Nernst electric 

 lamp with a one-ampere filament most suitable ; incandescent 

 gas is also very good, but the exposures become very long when 

 the magnification gets rather high. As it is necessary that each 

 picture should have an identical exposure, a time of the day must 

 be chosen when the pressure is steady. About midday and nine in 

 the evening are most suitable in this respect. I have also tried 

 limelight and the open arc. With limelight it is very difficult to 

 get equalised exposures ; pitting of the lime generally gives trouble 

 in the middle of an exposure. The open arc is beyond most 



