86 PRACTICAL PHOTO-MICROGRAPHY. 



experience, give little or no color which is not in the object, 

 and we never, with these lenses, find fringes of color. But we 

 find the focusing as easy with one glass as the other ; it is only- 

 after development that the superiority of the apochromatic 

 glasses shows itself unmistakably. 



When we use " Kamsden " or " Aplanatic " focusing ocular 

 color is often seen in the objects, but we must not attribute that 

 to the objective. If this color is objected to, a very low-power 

 ocular made by Zeiss, and called a " Searcher Eye-piece," may 

 be sunk in & plaque of wood, that is to say, the eye-piece may 

 be thrust through a hole in the wood to such a distance that 

 when the wood occupies the place of the ground glass or sensi- 

 tive plate, the diaphragm of the eye-piece occupies the critical 

 plane where the image is to fall on the sensitive plate. Several 

 holes may be bored in the plaque of wood, and the eye-piece 

 may then be moved from hole to hole. 



Under the next set of examples we may put a very large 

 series of objects always overlooking, for the present, the pho- 

 tographic difficulties of color to which an entire chapter is 

 allotted such objects, for instance, as the easier diatoms ; phy- 

 siological, histological and pathological subjects ; insect structure 

 and the larger bacteria where no minute structure is to be 

 shown, as flagella. For all such subjects where the magnifica- 

 tion required is from forty diameters upwards, the substage 

 condenser with bull's-eye may be used, and the focus in such 

 cases should be general rather than critical. Below forty 

 diameters the writer avoids the use of a magnifying glass for 

 focusing, believing that a better general focus is obtained with- 

 out the Kamsden; he admits, however, that his eyesight is 

 possibly abnormally sharp. As soon as the power used is 

 sufficiently high to magnify the focused flame-image so as to 

 make it cover the whole field to be photographed ; i. e., in all 

 magnifications over say 400 diameters, the writer always dis- 

 penses with the bull's-eye. As already pointed out, the size of 

 the flame-image on the field depends in the first place on the 

 focus of the condenser, in the next place upon the combined 

 power of objective and ocular. As the writer progresses in 

 experience he uses the bull's-eye less and less in his work. In 



