Optical Axis, an imaginary line which passes 

 from the center of the eyepiece through the centers of 

 the body, objective, stage and substage to the mirror. 

 Whatever lies in it is said to be centered. 



Object, that which is examined. 



Slide or Slip, a plate of glass upon which the 

 object is placed or mounted ; the prevailing standard 

 being 3 inches long by i inch wide. 



Cover Glass, a thin piece of glass cut circular 

 or square, which is placed upon the object, either for 

 flattening or preserving it, or both. The thickness 

 varies from 1-50 to 1-250 inch and this variation has 

 a very important bearing on the optical effect of the 

 microscope. 



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 makers, it has been almost entirely superseded by the 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 adjustment were applied, as near as 

 consistent with the construction, to the body tube; 

 whereas in the Ross form they were placed at the back 

 or more distant point in the instrument, thus increasing 

 by means of the connecting arm the faults which might 

 exist in the adjustment. 



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