MICROSCOPIC OBJECTS. 193 



and large field of view and all the optical properties of 

 the instrument what they should be,) we can only gain 

 £in advantage one way to lose it another, if we kick 

 against these laws of optics. 



I once attempted to make a megaloscope as foUows : — 

 the object-glass was 2\ inches focus and f of an inch in 

 aperture ; the body, I think, 8 inches long or less ; the 

 two field-glasses were each 3 inches focus and l^ in. in 

 aperture, placed in contact. The figures were plano- 

 convex, with their plane surfaces next the eye; the eye- 

 glass was one-inch focus, plano-convex, the plane side 

 next the eye, and only one inch distant from the anterior 

 field-glass, which was consequently in its focus. Now 

 this construction was excellent in some points : its field 

 of view was very large — 70° I think — quite free from 

 distortion and indistinctness at the edges ; and, as the 

 anterior focus of the objective part was longer than the 

 posterior, the image was a diminished instead of a mag- 

 nified one, and the power of the entire instrument very 

 low, being, if I rightly recollect, only half of that of 

 the eye-glass, or equal to a lens of 2 inches focus. 



What were its faults ? — Why, first, the anterior field- 

 glass being in the focus of the eye-glass, aU the imper- 

 fections, scratches, and particles of dust, &c., on the 

 former were visible ; secondly, the doubling of the field- 

 glasses (for no single one would answer) caused the vision 

 of opaque objects to be very dull and unsatisfactory, as is 

 always the case with double field-glasses and eye-glasses. 

 (We may double and treble object-glasses without doing 



