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190 OPTICAL PROJECTION objective-changers, costing 10s. each lens, and each of which is adjusted by screws once for all, so as to centre its own objective exactly in the axis of the substage condenper. 94. Projection with the Compound Microscope.—When- ever a substantial microscope with focussing and centering substage is at command, very good occasional projections can be made with it by the aid of a simple lantern attachment, embodying the foregoing arrangements as far as the concave lens P, which delivers from the lantern a brilliant parallel beam to the ordinary substage. It only needs for the sub- stage to have added to it a tube-screen to receive the beam and prevent stray light scattering, while a flat plate to prevent scattering is also screwed on the nose of the microscope, which will itself furnish all else that is required. I have designed such an arrangement, which is perfectly effective, at a small cost. For permanent work, however, a complete and solid projection instrument will be found more satisfactory. 95. The Demonstration Image. — It must never be forgot- ten that the projecting microscope produces its image under peculiar and distinct conditions, which interpose some peculiar difficulties. The first difficulty lies in the enormously greater amplification necessary. This does not produce a proportionate effect upon the spectator, owing to his greater distance from the image ; but, on the other hand, any errors in the objective, with their consequent woolliiiess in the image, are magnified proportionately. A power of 1,200 diameters -is considered very high on the compound microscope, in ordinary work; but is a very moderate power for screen work. The apparent depth or thickness of the slide is magnified also in the same proportion ; and cannot be counteracted by constant changes of focus as in private work, because the whole audience must see the whole at once. Hence * depth of focus ' is a very real matter in lenses for projection. The second difficulty is a consequent deficiency of light, which makes it specially difficult to exhibit images of trans- parent objects, such as diatoms. In these objects there is no