38 THE MICEOSCOPE. 



colors, it will be observed, is the reverse of that described in 

 treating of the common compound microscope (Fig. 12), in 

 which the single object-glass projected the red image beyond 

 the blue. The effect just described, of projecting the blue im- 

 age beyond the red, is purposely produced for reasons presently 

 to be given, and is called over-correcting the object-glass as to 

 color. 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 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 uncolored 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 colored 

 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 re- 

 quires; 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 jsum of the dis- 

 tances between the red and 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 colors 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 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 Huyghenean eye-piece pointed out by Boscovich. This 



