152 OPTICAL PROJECTION 



mechanical stage. The average * Shopticiau ' a very different 

 being from the more serious optician is very fond of an 

 instrument with as many screws, and racks, and 'adjust- 

 ments ' as he can possibly crowd upon it. Many a purchaser 

 is fond of the same ; and by all means let the one sell as much 

 as he can, and the other buy as much bright brass as he likes. 

 But the serious worker is of another sort ; and him I would 

 warn that all useless lumber of the kind is, to an ordinary 

 operator in the dark, simply a source of embarrassment and 

 distress. I have worked too long at this subject not to be 

 familiar with all manner of stages ; and affirm without hesita- 

 tion, that a mechanical stage is not only useless, but a positive 

 hindrance, for all but a limited class of work which will be 

 mentioned in due course. 



An attachment of this kind will do a great deal of really 

 interesting work, including a large range of that fascinating 

 modern pursuit, photo-micrography. The great bulk of 

 ordinary pond life is within its powers, the beautiful volvox can 

 be shown nicely, and a very respectable exhibition can be 

 made of the circulation of the blood in a frog's foot, &c. 



Leaving details respecting serious microscopic exhibitions 

 for more full treatment in Chapter XIII., it is only needful to 

 add here, that in all attempts at such, special care must be 

 taken in centering the light, as a very slight deviation from 

 the optical axis will cause a great falling-off in the results. 

 It is also needful that the lime-tray should move truly to and 

 fro, parallel to the optic axis of the lantern ; because at every 

 change of power, or whenever the operator sees that the 

 effect is not what he knows the instrument to be capable of, 

 he should try, by moving the lime-tray a little, backward or 

 forward, whether he cannot get a better illumination. Very 

 often he will find that he can ; and, generally speaking, the 

 lime will have to be rather more drawn back as the powers 

 get higher. If the tray does not slide truly, therefore, the 

 lime cannot be truly axial in both positions. 



