CH. I'HL] PHOTO-MICROGRAPHY. 185 



As the difficulties of photo-micrography are so much greater than of ordinary 

 photography, the advice is almost universal that no one shonld try to learn photo- 

 graphy and photo-micrography at the same time, but that one should learn the 

 processes of photography by making portraits, landscapes, copying drawings, etc., 

 and then when the principles are learned one can undertake the more difficult 

 problem of photo-micrography with some hope of success. 



The advice of Sternberg is so pertinent and judicious that it is reproduced : 

 "Those who have had no experience in making photo-micrographs are apt to ex- 

 pect too much and to underestimate the technical difficulties. Objects which 

 under the microscope give a beautiful picture, which we desire to reproduce by 

 photography, may be entirely unsuited for the purpose. In photographing with 

 high powers it is necessary that the objects to be photographed be in a single plane 

 and not crowded together and overlying each other. For this reason photograph- 

 ing bacteria in sections presents special difficulties and satisfactory results can only 

 be obtained when the sections are extremely thin and the bacteria well stained. 

 Even with the best preparations of this kind much care must be taken in selecting 

 a field for photography. It must be remembered that the expert microseopist, in 

 examining a section with high powers, has his finger on the fine adjustment screw 

 and focuses up and down to bring different planes into view. He is in the habit of 

 fixing his attention 011 the part of the field which is in focus and discarding the 

 rest. But in a photograph the part of the field not in focus appears in a promi- 

 nent way which mars the beauty of the picture." 



APPARATUS FOR PHOTO-MICROGRAPHY. 



\ 324. Camera. — For the best results with the least expenditure of time one of 

 the cameras especially designed for photo-micrography is desirable but is not by 

 any means necessary for doing good work. An ordinary photographic camera, 

 especially the kind known as a copying camera, will enable one to get good results, 

 but the trouble is increased, and the difficulties are so great at best, that one would 

 do well to avoid as many as possible and have as good an outfit as can be afforded. 



The first thing to do is to test the camera for the coincidence of the plane occu- 

 pied by the sensitive plate and the ground glass or focusing screen. Cameras even 

 from the best makers are not always correctly adjusted. By using a straight edge 

 of some kind, one can measure the distance from the inside or ground side of the 

 focusing screen to the surface of the frame. This should be done all around to see 

 if the focusing screen is equally distant at all points from the surface of the 

 frame. If it is not it should be made so. When the focusing screen has been ex- 

 amined, an old plate, but one that is perfectly flat, should be put into the plate 

 holder and the slide pulled out and the distance from the surface of the plate 

 holder determined exactly as for the focusing screen. If the distance is not the 

 same, the position of the focusing screen must be changed to correspond with that 

 of the glass in the plate holder, for unless the sensitive surface occupies exactly 

 the position of the focusing screen the picture will not be sharp no matter how 

 accurately one may focus. Indeed, so necessary is the coincidence of the plane of 

 the focusing screen and sensitive surface that some photo-micrographers put the 

 focusing screen in the plate holder, focus the image and then put the sensitive 

 plate in the holder and make the exposure (Cox). This would be possible with 

 the older forms of plate holders, but not with the double plate holders mostly used 

 at the present day. 



