or blackened. From the constant friction of mov- 

 ing the slide around, the lacquer or artificial 

 blacking becomes worn and in time the stage 

 assumes a shabby appearance. To overcome this, 

 glass has been used with more or less success, but 

 in recent years vulcanite has come into use and 

 has proven very successful. The peculiar gritty 

 feeling due to small particles of dust between the 

 stage and slide is not so noticeable as on a metal 

 surface and it is not affected by acids or alkalies 

 and will therefore retain its neat appearance 

 almost indefinitely. 



Revolving Stage. While in the largest num- 

 ber of instruments the stage is fixed and either 

 round or square, there are others which are revolv- 

 ing, that is, may be revolved around the optical 

 axis. These are absolutely necessary in the 

 examination of crystals and rock sections and are 

 then arranged with graduations around the edge 

 a series of divisions reading to degrees or fractions 

 of them by means of which the angles of the 

 objects are measured. As a slight deviation of 

 the center of revolving motion from exact coinci- 

 dence with the optical axis will cause the object to 

 swing out of the field, centering screws are pro- 

 vided by which this error can be quickly corrected. 

 It sometimes occurs as the stage is revolved, that 

 an object at the edge of the field, which is in focus, 

 gradually becomes indistinct, showing poorest at 



