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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