The Sub-Stage 17 



THE SUB-STAGE OR FITTING UNDER THE 

 STAGE TO CARRY CONDENSER, ETC. 



THE SUB-STAGE. This consists of a tube which should 

 be 1-527 inches = 38'786 millimetres, or, roughly 1J inches 

 full, internal diameter termed the ' Society's size.' It 

 carries illuminating apparatus for condensing the light 

 on the object, the polarizing prism, and other apparatus, 

 referred to on a later page. It is adjusted in a vertical 

 direction to and from the under surface of the stage by 

 means of a rack and pinion, and the ring carrying the 

 apparatus is mounted in an outer collar provided with 

 screws, by means of which the condenser, or other 

 apparatus, can be made exactly central with the objective 

 with which it is working. This central fitting is made to 

 rotate by rack and pinion in some instances for using the 

 polarizer, etc., but this is so rarely needed that it is hardly 

 necessary except for special work. It is essential that the 

 sub-stage should be substantially made, as it is a most 

 important fitting, often too little appreciated. For the 

 sake of economy some makers fit the sub-stage without a- 

 rack-work, it merely being designed to slide up and down 

 in the dovetailed fitting; this is neither desirable nor 

 convenient. A fine adjustment, to permit of the con- 

 denser being focussed in the most exact manner, is often 

 provided with the best stands, and it is exceedingly con- 

 venient and of very great importance where high-power 

 work is intended to be done. Often it is wished just to- 

 alter the focus of the sub-stage condenser very slightly. 

 In attempting so to do the tension on the milled head of 

 the rack-work is apt to cause vibration, so that the best 

 point of adjustment cannot be at once observed. By 

 communicating this small amount of motion with the fine 

 adjustment the focus is obtained to a nicety. It is also- 

 especially convenient where a number of specimens have 

 to be examined. The varied thicknesses of the slips neces- 

 sitate a slight readjustment of the condenser in each 



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