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 



