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very useful for such comparatively large objects as Mycetozoa, 

 Foraminifera, etc. With regard to illumination, the lecturer 

 preferred incident light. Stereo-photomicrographs taken by 

 transmitted light seemed to be very unsatisfactory, through 

 diffraction and shadow effects. Objects to photograph well from 

 a stereoscopic point of view should be full of detail in every part. 

 Dealing with the illuminant, the Nernst electric lamp with one 

 ampere filament was thought most suitable, although incan- 

 descent gas is good for moderate magnification. With limelight 

 it is difficult to get equalised exposui-es, as pitting of the lime 

 frequently gives trouble. The open arc is not generally suitable, 

 but enables very short exposures to be made. Using an open 

 arc taking fifty amperes, fully exposed negatives had been secured 

 ^vith an exposure of two seconds of some aecidia on nettle- 

 leaf X 67. With such a very powerful light, great care is 

 needed to avoid burning the specimen, and a deep cooling 

 trough is always used. Referring to the Lumiere Autochrome 

 plates, the speaker said it was a matter for regret that these 

 wonderful plates were unsuitable for stereoscopic work. The 

 trouble is not so much in the length of exposure as in the 

 patchiness of the screen. It is found impossible in practice to 

 mix the starch-grains, of which the tricolour filter is composed, in 

 accordance with theoretical requirements ; a number of grains 

 of the same colour will persistently adhere to each other, these 

 forming distinct colour blotches, which float in space over the 

 picture proper, when viewed in the stereoscope. As photomicro- 

 graphic work is usually done with an artificial source of light, 

 "the usual autochrome filter is unsuitable. A special filter is 

 made for use with the electric arc, but the lecturer had not used 

 it with much success, the photographs taken being too brown 

 all over. This might possibly be due to variation in composition 

 of the carbons used in the arc, such variations creating small 

 differences in the colour composition of the light, to which these 

 plates are exceedingly sensitive. A very good method of working 

 is to convert the light from a Nernst lamp to theoretical daylight 

 by a special blue filter made for the purpose, and then use the 

 usual autochrome daylight filter. This causes very long ex- 

 posures, but the results, some of which the lecturer showed, are 

 well worth the extra trouble involved, as the colour rendeiing is 

 .as accurate as could possibly be wished. 



