HIGH-POWER OE CRITICAL PHOTO-MICROGRAPHY 



143 



see Fig. 12, Plate V.; but the pronounced superiority of the apochromatic becomes 

 more immediately apparent in the fine markings of faint diatoms such as the Amphi- 

 pleura pellucida, the Surirella gemma and the Coscinodiscus asteromphalus. 



In making nikko prints do not over expose : well develop to get plenty of com- 

 parison and contrast. Keep the developing solutions very strong to get black prints 

 and not brown ones. 



(ii) Photographing Bacteria. — Bacteria in their natural uyistained state are 

 exceedingly small, and for the most part colourless and structureless bodies of proto- 

 plasmoidal matter assuming all manner of shapes and having such a high refractive 

 index as to be most difficult to see — even with a magnification of 1000 diameters, and 

 still more difficult to photograph. 



Although these little organisms can be artificially stained by solutions of many 

 substances, still, each variety seems to have a more or less well-marked selective 

 power of absorbing the colouring matter of one or two special dyes in preference to 

 that of any other. Hence, as the photo- micrographer meets with this type of work 

 he must expect not only all varieties of shape, but all varieties of colour also. 



The difficulty noticed by all photographers in obtaining a well-contrasted print of 

 these little bodies depends solely upon the fact that the dyes with which the organ- 

 isms are stained do not affect the photographic plate in the same manner and to the 

 same extent as they do the eye. For example, a slide may be a very excellent one 

 from a bacteriological point of view, and owing to the selective colouring of the 

 organism as compared with that of the background, may be of a most impressive 

 nature, yet it will not yield an equally contrasted print. On consideration, the 

 reason for this apparent anomaly is not far to seek. Contrast visually was obtained 

 by differentiation of colour rendering between the bacillus and the background, 

 whereas in the photograph it must depend upon lohat effect each colour can cause on 

 the 'plate. Put another way, contrast visually is due to duality of colour rendering, 

 whereas in the print it alone depends upon what amount of differentiation in terms of 

 black and white the colours employed as stains have been able chemically to produce — 

 by deposition of silver — in the emulsion which covers the plate. 



The aim, then, of the operator must be to increase the difference in these deposit 

 ratios as far as he possibly can. 



To understand how this can be effected, the attention of the reader is directed 

 to the following : — 



I. Colour sensation to the eye is dependent upon the wave length of the light 

 employed. The longest wave gives rise to the red sensation, and the shortest to the 



