The effect described, that of projecting the blue image beyond the red, over-correcting the object-glass as to colour, is purposely produced; it is also seen that the images _b b_ and _r r_ are curved in the wrong direction to be seen distinctly by the convex eye-lens; this then is a further defect of the compound microscope made up 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 here shown, giving 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 produce an almost colourless image to the eye. The chromatic aberration of lenses has been clearly explained in a previous chapter. But let it be supposed that the object-glass had not been over-corrected, that it had been perfectly achromatic; the rays would then have appeared coloured as soon as they had passed the field-glass; the blue rays of the central pencil, for example, would converge at _b′′_, and the red rays at _r′′_, which is just the reverse of what is required of the eye-lens; for as its blue focus is also shorter than its red, it would require that the blue image should be at _r′′_, and the red at _b′′_. This effect is due to over-correction of the object-glass, which removes 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 several colours coincide, so far as focal distance is concerned, as the rays pass the eye-lens. So that while they coincide as to distance, they differ in another respect--the blue image is rendered smaller than the red by the greater refractive power of the field-glass upon the former. 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 accidental effect in the Huyghenian eye-piece was pointed out by Boscovich. The separation into colours of the field-glass is like the over-correction of the object-glass--and opens the way to its complete correction. If the differently-coloured rays were kept together till they reached the eye-glass, they would still be coloured, and present coloured images to the eye. The separating effected by the field-glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to its spherical figure, the refractive power is less than at the margin, so that spherical error of the eye-lens may be said to constitute a nearly equal balance to the chromatic dispersion of the field-lens, and the blue and red rays _l′_ and _l′′_ emerge nearly parallel, presenting a fairly good definition of a single point to the eye.