18 Modern Microscopy 



instance, and this can be very quickly done if a fine adjust- 

 ment be fitted to the sub-stage. Further, the modern sub- 

 stage condensers possess such large apertures that their 

 exact adjustment becomes equal in importance to the 

 precise focusing of the objective. 



Where a microscope is provided with a sub-stage it is 

 necessary to ascertain if- it will centre with the objective 

 by means of its screws ; this should be done in the same 

 manner as described below for the ' under-fitting,' and the 

 centring screws turned. Also, it is very important that 

 when the sub-stage is racked up or down it should main- 

 iain its centre with the optical axis. But few instruments 

 will stand this test ; in consequence of untrue mounting 

 or build the sub- stage goes out of centre slightly in some 

 cases, considerably in others. There ought to be absolute 

 truth if everything is square, and any great deviation in 

 this respect should call for rectification. If a fine adjust- 

 ment be fitted to the sub-stage, it may be tested by using 

 ihe upper surface of the fitting as a stage and placing the 

 object on it; this may be made to adhere with a little 

 tallow or grease. An objective of medium power will prob- 

 ably not focus, the sub-stage being too far away. The 

 nosepiece end of the microscope must therefore be length- 

 ened ; for this purpose remove the prism from an analyzer 

 fitting, and use this fitting as a lengthening adapter. The 

 object is then viewed in the usual way. 



In the construction of the sub- stage once again the 

 Continental microscopes are not for a moment to be com- 

 pared with their English contemporaries. There is not a 

 single maker of microscopes on the Continent who provides 

 -centring screws to his sub-stage. It is simply impossible 

 to do good work without this convenience. There is hardly a 

 worker nowadays who does not have objectives by more than 

 one maker, and it will be found that these have different 

 centres. Continental opticians maintain that their method 

 simplifies work, for the condenser is centred once for all 

 -to the objectives that are supplied with the instruments. 



