220 PHOTO-MICROGRAPHY \_CH. VI11 



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

 expect 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 photo- 

 graphed be in a single plane and not crowded together and overlying each 

 other. For this reason photographing 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 microscopist, 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 on 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 prominent 

 way which mars the beauty of the picture." 



APPARATUS FOR PHOTO-MICROGRAPHY 



I 296. 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 indispensable for doing good work. An ordinary photo- 

 graphic 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 (Fig. 170). 



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

 occupied 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 examined, an old plate, but one that is perfectly 

 flat, should be put into the plate holder and the slide pulled out and the dis- 

 tance from the surface of the plate holder determined exactly as for the focus- 

 ing 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. In- 

 deed, 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. 



