MICROSCOPIC OBJECTS. 199 



in situ, the other two being taken out. It (the shallow 

 object-glass) need never be removed, except when the 

 front glasses are wanted to act separately. The stop 

 should also be left in the tube when the shallow object- 

 glass is in action : it prevents extraneous rays not pro- 

 ceeding from the object under examination, and those 

 reflected from the inner surface of the tube, from being 

 refracted, I need scarcely observe that the rays from 

 near objects not being parallel but divergent, the said 

 stop, being pretty near the focus, does not cut off any of 

 the aperture of the object-glass or reduce the size of the 

 visual pencil. 



The object-glasses make delightful hand- magnifiers, 

 or, in strict language, achromatic microscopes : their 

 inner surfaces being cemented and their aberration of 

 both kinds neutralized, we see through them as if we 

 were looking through a piece of plane glass.* 



MECHANICAL ARRANGEMENTS. 



It is obvious that a Megaloscope may be mounted in 

 the same manner as any other engiscope; only it will 

 require a very strong and massive stand. I am disposed 

 to think, myself, that one chiefly made of wood answers 

 well enough ; but of course everything of this sort which 

 can be executed of wood, can be executed in metal at 

 pleasure, reducing the dimensions in the ratio of the 



* I ought to state, for the information of the reader, that the 

 optical part of this Megaloscope was made at Mr. Pritchard's 

 manufactory, 1G2, Fleet-street, London. 



