20 Modern Microscopy 



their ' Edinburgh Students ' and ' Fram ' Instruments, 

 make many of the fittings interchangeable, so that a person 

 who starts with an under - fitting, and subsequently ex- 

 periences the necessity of having a sub-stage, can himself 

 remove one fitting and replace it by the other. W. Watson 

 and Sons have lately adopted a form of centring under- 

 fitting; it is inexpensive and very considerably increases 

 the efficiency of a student's microscope. It can be fitted 

 to almost any microscope. 



The great convenience will be found in many instru- 

 ments of being able to swing the sub-stage aside out of the 

 optical axis of the instrument on a hinge-joint fitting. It 

 saves much time to students, especially where two or three 

 powers are constantly being interchanged, and the con- 

 denser may not be required for all of them. Where this 

 arrangement exists it should be adapted in a workman-like 

 and substantial manner, and a proper support given to 

 the fitting when in the optical axis to make it perfectly 

 rigid. 



The choice with regard to a sub-stage would therefore 

 be— 



In a first-class microscope : Sub-stage, having rack-work 

 and fine adjustment for focusing, and provided with 

 facilities for centring ; rackwork rotation, if for examina- 

 tion of crystals or for petrology. 



Second-class instrument : Sub-stage, having rack-work 

 and centring adjustments, and means of lifting aside out 

 of the optical axis. 



Student's instrument : The same as the second-class, or 

 with the plain under-fitting ; the latter, preferably with 

 centring arrangement. In any ease it is imperative that 

 it shall be of the ' Universal ' size. 



