28 PHOTOMICEOGRAPHY 



a large plane stage is required. To be perfectly finished 

 a rotating stage should be graduated and fitted with 

 centring screws, and a complete mechanical stage would 

 have milled heads for the rotating as well as the rectangular 

 movements. Such refinements are only found on the 

 most expensive stands, and for most purposes are not 

 required. The aperture in the stage ought to be of 

 sufficient size to allow of the use of any condenser that 

 may be required, and there must be no tendency to foul 

 the latter at any point of the traverse of a mechanical 

 stage. 



The Substage. The fittings situated beneath the stage 

 for carrying the condenser and its accessories, should 

 comprise suitable movements for placing the condenser 

 in the best position for illuminating the object, and 

 these are almost as important in a photomicrographic 

 stand as the coarse and fine adjustment of the tube. 

 The simplest form is a fixed sleeve, in which the con- 

 denser slides, but this is not adequate for accurately 

 focussing the latter. The better microscopes have a 

 mechanical focussing substage consisting of a ring or 

 sleeve raised and lowered either by a quick-acting 

 lateral screw (Fig. 4), or by a rack and pinion (Fig. 17). 

 The rack-and-pinion arrangement is to be preferred, but 

 the quick-acting lateral screw is a fairly efficient substitute 

 and usually fitted to the cheaper stands. Some of the 

 most complete substages have a fine adjustment as well 

 as the rack and pinion, but this is not often required. 



An Iris diaphragm is an essential part of an illuminating 

 system, and is either fitted to the condenser mount 

 itself, or forms an independent part of the substage. 

 An iris is also fitted in the thickness of the stage of some 

 microscopes, or supplied as a cylinder fitting over the con- 

 denser (Figs. 15 and 16), and is a most useful accessory. 



The substage is sometimes made to swing out of the 

 optical axis when racked down to its limit, and this is 



