CHAPTER XVIII 

 PHOTOGRAPHY 



Difficulties. — Theoretically, it would seem that photog- 

 raphy should aid microscopy perhaps almost as much as 

 it aids astronomy. Practically, this does not happen. 

 Probably most of the important discoveries with the micro- 

 scope have been illustrated by camera drawings, not by 

 photographs. The reasons for this are: the restriction 

 of the microscopical photograph to one plane, which makes 

 it best suited to such objects as blood films, and smear 

 preparations of bacteria; the waste of time entailed in 

 setting up the microscope and camera; the usual lack of 

 critical sharpness in the high-power photographs, except 

 when taken with a special projection eyepiece; the time 

 spent in making trial exposures ; the use of a too low working 

 aperture, that is, too small a condenser cone; glare through 

 not diaphragming the source of light; and the need for 

 increasing the contrasts in the picture. 



Errors in Photography with the Microscope. — To obtain 

 optimum photographs, certain laws of optics must be 

 closely conformed to: 



1. The distance of the objective from the object, when a 

 photograph is to be taken, should be the same as when the 

 object is in focus and viewed through the eyepiece (with 

 correct tube length). This prevents any but small photo- 

 graphs being taken with the objective alone; for the 

 photographic plate must be put where the image falls 

 when the eyepiece is removed (after focusing), without 

 altering the focus. 



2. A photograph cannot be taken with the focused 

 eyepiece in situ without raising the focus of the objective 

 and so increasing the objective distance. (This may be 

 avoided by focusing by increasing the tube length, and not 

 raising the objective.) 



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