PRACTICAL PHOTOMICROGRAPHY. 



83 



A (Fig. 29) shows a substage condenser and an objective 

 focused on the same point, and the aperture of both equal and 

 fully utilized. On looking down the microscope tube the lens 

 will be seen filled with light as at C. 



B shows a condenser and an objective still focused on one 

 point, but having their apertures cut down by a stop in the 

 condenser ; the back lens, examined as before, will present the 

 appearance of D. 



FIG. 29. 



These remarks and quotations, though, perhaps, not in their 

 proper order here, are to be specially noted in every case where 

 a bull's-eye is used. The bull's-eye, as already stated, should 

 be on a stand with a heavy base, and in order to focus the con- 

 denser when the bull's-eye is to be used, the following steps 

 may be taken. The objective being focused, the condenser 

 and light centered, the edge of the bull's-eye is advanced in 

 front of the light and the image of the metal rim of the bull's- 

 eye is focused on the object by means of the condenser. At 

 the same time the worker must take care that the segment of 



