84 PRACTICAL PHOTO-MICROGRAPHY. 



the circle of the bull's-eye seen on his field is vertically central, 

 i. e., the imaginary centres of bull's-eye and field should be in 

 one line. This is difficult to explain, but will be understood 

 on experiment. Lastly, when the field is illuminated, the 

 bull's-eye being focused and in its place centrally, the worker 

 is to look down his tube and compare what he sees with our 

 figures on pages 82 and 83. If the entire area of the back 

 combination is filled with light, we are utilizing all the aper- 

 ture of our objective, and it may be said that, in many cases 

 where it is desirable to utilize every fraction of aperture that 

 our objective possesses, the use of the bull's-eye is to be recom- 

 mended. In many cases the use of our entire aperture intro- 

 duces photographic difficulties, but these must be overcome. 

 If a photograph is wanted of the " general appearance " 

 of an object, it is well to cut down the aperture much more 

 than would be permissible where a scientific photographic 

 representation is required. We are aware that this is heresy 

 to some old workers. 



Example No. 3. A " critical image," with a low power, a 

 condenser, no bull's eye. Subject : " Test hairs " on a blow- 

 fly's proboscis. Objective: A two-thirds, one-half, or one- 

 quarter inch of highest attainable aperture ; this test for a one- 

 quarter inch being, as a rule, too easy. Projection ocular 

 used ; magnification from 150 to 400 diameters. See Plate I, 

 fig. 1. 



A and B, centre condenser and light as before. (7, for the 

 two-thirds, or one-half, or four-tenths objective, the substage 

 condenser may be used without its front hemisphere, unless 

 with its front it has a very low angle, or unless it is not achro- 

 matic. Two things must always be observed with regard to 

 the condenser ; first, it must have sufficient angle to fill the 

 objective ; second, whatever its angle be, if its angle is greater 

 than that of the objective, it must be stopped down till the 

 angles are nearly equal. This time the object is focused care- 

 fully with the objective, and then the light is most carefully 

 focused on the part of the u tongue " to be photographed. The 

 hairs will now be seen more or less elongated, according as*the 

 correction of the objective happens to be more or less accurate. 



