The Sub-Stage. 28 



the objects, polarizing prism, etc., referred to on a later 

 page. It is adjusted in a vertical direction towards the 

 under surface of the stage by means of a rack and pinion, 

 and the ring carrying the apparatus is provided with screws, 

 by means of which the condenser, etc., 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 this should be substantially made, as it 

 is a most important fitting, often too little appreciated. 

 For the sake of economy, some makers fit a sub-stage with- 

 out a rackwork, merely sliding up and down in the dove- 

 tailed fitting ; this is not desirable. If a sub-stage be had, 

 it should have a rackwork, or nearly all its value is lost. 

 A fine adjustment, so that the most exact focus can be 

 obtained with the condenser, is often provided with the best 

 stands, and is very useful if high-power work is intended to 

 be done. It is surprising the convenience this fine adjust- 

 ment affords. Often it is wished to just altei the position 

 of the light and sub-stage condenser a little. In so doing 

 the tension on the sub-stage milled head is apt to cause 

 vibration, so that thle 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. 



In the cheaper instruments, instead of the sub-stage as 

 above described, an ordinary plain tube is fitted into the 

 under part of the stage, and in this tube the condenser or 

 other apparatus is moved up and down to focus. This 

 must be truly centred with the optical tube, and it is well 

 to test it by putting a small diaphragm into the under tube, 

 and with an objective in the body to focus the diaphragm. 

 If it be not central its practical importance is annulled. 

 The additional convenience of the centring sub-stage is well 

 worth the extra outlay. The instrument in Fig. 1 has a 

 rackwork ; Fig. 2 has a plain tube only. 



