HUYGHENIAN EYE-PIECE. 



49 



The effect just described, of projecting the blue 

 image beyond the red, is purposely produced for 

 reasons presently to be given, and is called over- 

 correcting the object-glass as to colour. It is to be 

 observed, also, that the images b b and r r are curved 



in the wrong direction to be distinctly seen by a 

 convex eye -lens, and this is a further defect of the 

 compound microscope of two lenses. But the field- 

 glass, at the same time that it bends the rays and 

 converges them to foci at &' V and r f /, also reverses 

 the curvature of the images as here shown, giving 



E 



