THE PROJECTION MICROSCOPE 185 



I however found some modification of the combination give 

 better results. 



All substage condensers fit easily from the front into a ring 

 of the standard R. M. S. U-inch substage gauge. Thus, any 

 substage apparatus approved by any microscopist, can be at 

 least tested for projection purposes, and, if successful, adopted. 

 A spot lens, which will do excellent dark-ground work on a 

 certain class of objects, is inserted in the same way. The barrel 

 of the instrument being now constructed open at the top, the 

 substage condensers can with equal facility be inserted or ex- 

 changed from the back of the ring, without interfering with 

 the slide or focussed objective; and this arrangement also 

 allows of a condenser with iris diaphragm being employed, 

 which is very useful in high-power work, as the best definition 

 can only be obtained with a cone of light of some particular 

 angle. 



The Stage s cannot be too simple for ordinary work, 

 even with high powers. A plain plate, with two simple clips 

 capable of holding either a glass trough or a slide, is to be 

 preferred, even for immersion lenses. One very good plan, 

 preferred in practice by the majority, is to make the stage 

 consist of a large rotating diaphragm plate, pierced with 

 different apertures, ranging from T \y inch, to a size sufficient 

 to allow of the condenser-fitting racking out for manipulation. 

 Another plan preferred by some, is to have a set of plates 

 pierced with different apertures, sliding in dovetailed grooves, 

 so as to be a hair's breadth below the surface of the outer 

 stage-plate. This pattern allows the aperture to be changed 

 without removing the slide. In practice, however, any other 

 than the largest aperture is seldom needed ; the higher power 

 substage condensers confining the pencil of light sufficiently. 



A mechanical stage is simply a nuisance for ordinary 

 projections. It does not cost much, but is only useful to two 

 classes of persons. The first consists of such as use the 

 instrument for photographic purposes (for which it is admirably 



