49 



distance from this to the substage or mirror does 

 not vary, except when made to do so. 



S. Diaphragm. This is a provision for stopping- 

 down or regulating the amount of light which 

 illuminates the object. 



Optical Axis. This is an imaginary line which 

 passes from the center of the eyepiece through 

 the centers of the body, objective, stage and sub- 

 stage to the mirror. Whatever lies in it is said to 

 be centered. 



Object. That which is examined. 



Slide or Slip. This is a thin plate of glass 

 upon which the object is placed or mounted, the 

 prevailing standard being 3 inches long by 1 inch 

 wide. 



Cover Glass. This is an extremely thin piece 

 of glass, round or sqtiare, which is placed upon 

 the object, either for flattening or preserving it, or 

 both. 



Classification of Microscopes. Until recently 

 microscopes were divided into two classes, the 

 Jackson and the Ross models. While the latter 

 was for many years very popular, particularly 

 with the English makers, it has been almost 

 entirely superseded by the Jackson form and with 

 good reason. In the former the means of adjust- 

 ing were provided, as near as consistent with the 

 construction, to the body or tubes, whereas in the 



