the diaphragm should be about that of the front lens 

 of the objective. For some objects and some objectives 

 this rule may be quite widely departed from; one must 

 learn by trial."* The concave mirror acts as a lens and 

 has a focus like a lens. It will often be found that by 

 carefully focusing the mirror, details will be brought 

 out clearly which otherwise would be but dimly seen. 



It may be found that in focusing up and down, the 

 image shifts slightly from right to left, or to and fro. 

 This may possibly be due to an imperfection in the 

 microscope, but if the instrument is in good repair, and 

 from any one of the reputable makers, the chances are 

 more than likely that the shifting is due to oblique light, 

 even though the mirror bar may be in the median line. 

 This is even more apparent with a condenser than with- 

 out it. Manufacturers are often condemned because of 

 a mistaken idea that the mirror bar in this position 

 means axial light. A slight turn of the mirror will stop 

 the shifting and give axial illumination. When there is 

 no lateral motion in focusing, the light is centered. 



Oblique Light. Some objects, such as diatoms, 

 rulings, etc., are better defined when oblique light is 

 used. This is accomplished without the condenser by 

 swinging the mirror out of the optical axis and so turning 

 it as to throw as much light as possible upon the object. 

 When the ocular is removed the image of the mirror will 

 be seen at one side o the center of the back lens of the 

 objective. When focusing a decided lateral motion of 

 the object will be noticed. 



ILLUMINATION WITH SUBSTAGE CONDENSER 

 Central Light. All of the better microscopes are 

 provided with a condenser fitted beneath the stage, which 

 brings parallel rays of light to a focus at a point above 

 *The Microscope. Gage. 



18 



