CONSTRUCTION OF THE MICROSCOPE. 49 



pass uiicoloured through the eye-glass. To render this 

 important point more intelligible, let it be supposed that 

 the object-glass had not been over-corrected, 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 r r 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 respect, 

 the blue images are rendered smaller than the red by the 

 superior 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 former repre- 

 senting the red, and the latter the blue rays. This is the 

 accidental 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 separation 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 sensibly 

 parallel, presenting, in consequence, the p'erfect defini- 

 tion of a single point to the eye. The same reasoning 



