200 



PHO TO- MICROGRAPH Y. 



[CH. VIII. 



circle, i. e. , the diaphragm of the ocular, is sharp, the resulting picture 

 will not be satisfactory. When the diaphragm is sharply focused on 

 the screen, the microscope is focused exactly as though no ocular were 

 present, that is, first with the unaided eye then with the focusing glass ; 

 the object should be in focus in the beginning. 



Fig. 157. Arrangement for Artificial Illumination. 



/. Lamp with metal chimney, easily made by rolling up some ferrotype plate 

 and making a slit-like opening in otie side. This opening should be covered by an 

 oblong cover-glass. A glass slide, being of considerable thickness, breaks too 

 easily. The lamp should have a wick about 30 mm. wide, so that the thickness of 

 the flame, if taken edgewise, will give an intense light. A wide fame also enables 

 one to get a larger image of the flame, and thus illuminate a larger object than as 

 though a small fame were used. 



2. Bull's-eye condenser on a separate stand. The engraving glass shown in Fig. 

 T 55> or the tripod magnifier {Fig. 154.) answers fairly. The Steinheil lenses are 

 still better. 



j. Screen showing image of the flame inverted. 



The lamp and bull's- eye stand are on blocks zvith screw-eyes as leveling screws. 



The exposure is also made in the same way, although one must have 

 regard to the greater magnification produced by the projection ocular 

 and increase the time accordingly ; thus when the X 4 ocular is used , 

 the time should be at least doubled over that necessary when no ocular 

 is employed. 



Zeiss recommends that when the bellows have sufficient length the 

 lower projection oculars be used, but with a short bellows the higher 

 ones. It is also sometimes desirable to limit the size of the field by 

 putting a smaller diaphragm over the eye lens. This would aid in 

 making the field uniformly sharp. 



