THE EYE-PIECE. 37 



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 ob- 

 served also, that the images b b and r r are curved in the wrong direc- 

 tion 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 

 b' b' and r r, also reverses the curvature of the images as there shown, 

 and gives them the form best adapted for distinct vision by the eye- 

 glass e e. The field-glass has at the same time brought the blue and 

 red images closer together, so that they are adapted to pass un coloured 

 through the eye-glass. To render this important point more intelli- 

 gible, let it be supposed that the object-glass had not been over-cor- 

 rected, that it had been perfectly achromatic; the rays would then 

 have become coloured as soon as they had passed the field-glass ; the 

 blue rays, to take the central pencil, for example, would converge at 

 b", and the red rays at r", which is just the reverse of what the eye- 

 lens requires; for as its blue focus is also shorter than its red, it would 

 demand rather that the blue image should be at r", and the red at b". 

 This effect we have shown to be produced by the over-correction of the 

 object-glass, which protrudes the blue foci b b as much beyond the red 

 foci rr as the sum of the distances between the red and the blue foci 

 of the field-lens and eye-lens ; so that the separation b r is exactly taken 

 up in passing through those two lenses, and the whole of the colours 

 coincide as to focal distance as soon as the rays have passed the eye- 

 lens. But while they coincide as to distance, they differ in another re- 

 spect, the blue images are rendered smaller than the red by the su- 

 perior refractive power of the field-glass upon the blue rays. In tracing 

 the pencil I, for instance, it will be noticed that, after passing the field- 

 glass, two sets of lines are drawn, one whole and one dotted, the for- 

 mer representing the red, and the latter the blue rays. This is the ac- 

 cidental effect in the Huyghenian eye-piece pointed out by Boscovich. 

 The separation into colours of the field-glass is like the over-correction 

 of the object-glass, it leads to a subsequent complete correction. For 

 if the differently coloured rays were kept together till they reached the 

 eye-glass, they would then become coloured, and present coloured 

 images to the eye; but fortunately, and most beautifully, the separa- 

 tion effected by the field- glass causes the blue rays to fall so much 

 nearer the centre of the eye-glass, where, owing to the spherical figure, 

 the refractive power is less than at the margin, that that spherical error 

 of the eye-lens constitutes a nearly perfect balance to the chromatic 

 dispersion of the field-lens, and the blue and red rays I' and I" emerge 



