MAGNIFYING POWERS. 169 



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 B' B' and R' R'i 

 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 uncoloured 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 pro- 

 duced by the over-correction of the object-glass, which pro-- 

 trudes the blue foci, B B, as much beyond the red foci, R R, as 

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

 field-lens and eye-lens ; so that the separation^ BR, 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 dis- 

 tance, they differ in another respect ; the blue images are ren- 

 dered smaller than the red by the superior refractive power of 

 the field-glass upon the blue rays. In tracing the pencil, L, 

 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 representing 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 dif- 

 ferently coloured rays were kept together till they reached 

 the eye-gla^s, they would then become coloured, and present 



