above that which is required in moving the object 

 about, the stability may be considered ample. In older 

 instruments the fault often occurred of making the stage 

 unnecessarily thick, which for present day require- 

 ments or with modern substage appliances would inter- 

 fere with the accomplishment of the best results. At 

 the present time the stages of instruments from reput- 

 able makers are of ample strength without undue 

 thickness although in cheap foreign products this is too 

 often not the case. It is of the greatest importance, 

 however, that the surface of the stage should be square 

 with the tube in all directions. Any deviation pro- 

 duces inferior optical results. In the better class of 

 instruments a vulcanite plate attached to the upper 

 part of the stage 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 much affected by acids or 

 alkalies and will therefore retain its neat appearance 

 almost indefinitely. 



Revolving Stage. While in the largest number 

 of instruments the stage is fixed and generally square, 

 there are others in which it is revolving, that is, may 

 be revolved around the optical axis. Such a stage is 

 an absolute necessity in the examination of crystals 

 and rock sections, for which pjurpose graduations in 

 degrees or fractions of degrees, by means of which 

 the angles of the objects may be measured, are pro- 



38 



