THE MICROSCOPE. 



KKBRUARY, 



1893. 



Number 2. (New Series.) 



PHOTOMICROGRAPHY. 

 ByF. A. Rogers, M. D., 



WORCESTER, MASS. 



^Continued from page j.] 



The most particular step in the whole process of photomicrog- 

 raphy, aside from lighting, is the focusing. We must not ex- 

 pect to focus *• prettv good " and that the plate or developer will 

 do the rest. If we expect to get a good sharp negative we must 

 be sure that a good sharp image is projected upon the plate, for 

 the plate will not lie, but will reproduce accurately the view 

 which has impressed it. 



The ground glass and plate must be in the same plane when 

 thev are exchanged ; in other words, the sensitive side of the plate 

 must occupv the identical place of the ground surface upon which 

 the image is focused, in order that we may obtain a good nega- 

 tive. This becomes more important as work is attempted with 

 higher powers, for then it is often difficult to determine when the 

 object is just in focus ; a magnifier or focusing glass now may be 

 used with great advantage, but this magnifies the blurred and in- 

 distinct outline on the apparently much roughened surface of the 

 ground glass. Instead of ground glass one may employ a nega- 

 tive which has been exposed to daylight and developed in the 

 usual way, the film upon the surface being excellent to focus 

 upon. Some coat the surface of the ground glass over with tal- 

 low or vaseline and claim that it improves it. But there is 

 another method which is excellent for focusing high powers. 

 Substitute a w r ooden slide for the ground glass, and let there be a 

 hole or several holes in it large enough to receive the ocular. 

 Have the slide of such a thickness that when the ocular is pushed 

 in as far as it will go the diaphragm shall lie in the same plane 

 as the surface of the ground glass. Insert it. and by this means 

 the focusing of a one-fifteenth inch objective becomes an easy 

 matter. This is a great improvement over the method of using 

 a ground glass, or any other surface to focus a high power objec- 

 tive upon. 



