CONSTRUCTION OP THE MICROSCOPE. 48 



uucoloured 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 6", 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 &". 

 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 <jf 

 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 V emerge sensibly 

 parallel, presenting, in consequence, the perfect defini- 

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



