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KNOWLEDGE OF PHOTOGRAPHY. 



Any one desiring to learn bow to make good photomicrographs must 

 procure a camera and learn how to make a good negative; it will not 

 do for him to press the button and let some one else do the rest; he can 

 not leai-n what a good negative is until be has made many and tested 

 their printing qualities. When any one is a fair judge of the sort of 

 lantern slide or print a negative will make he can then make a good one, 

 and when he can at morning or at noon, on a clear or a cloudy day make 

 a landscape negative and print it on glass or paper so well that bis print 

 compares favorably with the best of its class in the market, he may begin 

 to experiment at photomicrography. He generally begins long before this 

 and always produces and often publishes work that he never would have 

 published could he have known Avhat others were doing. Almost every 

 photomicrographer has thrown away crop after crop of negatives which 

 he formerly cherished as the best producible. At this stage he either 

 quits or goes into a thorough study of the principles of photography on 

 the simplest outdoor work; the production of high-power photomicrographs 

 is the most difficult problem in photography and can only be done by good 

 photographers who have had much experience also in low power work. 



THE OBJECT TO BE PHOTOGRAPHED. 



The photography of diatoms has flourished as a scientific fad for 

 years. It is a special line of photography, calling for special illumination 

 and specially prepared objectives; it calls for resolution, while general 

 histological work requires penetration. It was for a long time a race 

 with instrument makers to see which could resolve the finest striations; 

 diatoms were used for test objects almost exclusively. It was gravely 

 argued that a microscope that was good for diatoms was good also for 

 other things in like proportion. Oblique illumination and blue light were 

 praised for the same reason. The comfortless purchaser was left to 

 reflect— having resolved a pleurosigma or an amphipleura— how few of 

 them he ever cared to resolve, and that blue light concealed what he 

 wanted to see. Every one easily admits, however, now that a diatom 

 can be photographed: and since the publication of Koch's Bakterienkunde 

 in 1889 and 1890 it has been granted that bacteria can be; they can be 

 made to lies so uniformly in one plane. Doubtless it will always remain 

 true that some things can be photographed better than others, and that 



