The peculiarities to be noted are the curvature, indistinctness, and false colouring of the image.
The curvature of the image is the least important of the three defects named—a fortunate circumstance, since this defect admits neither of remedy nor modification. The image of a distant object, instead of lying in a plane, that is, forming what is technically called a flat field, forms part of a spherical surface whose centre is at the centre of the object-glass. Hence the centre of the field of view is somewhat nearer to the eye than are the outer parts of the field. The amount of curvature clearly depends on the extent of the field of view, and therefore is not great in powerful telescopes. Thus, if we suppose that the angular extent of the field is about half a degree (a large or low-power field), the centre is nearer than the boundary of the field by about 1-320th part only of the field's diameter.
The indistinctness of the image is partly due to the obliquity of the pencils which form parts of the image, and partly to what is termed spherical aberration. The first cause cannot be modified by the optician's skill, and is not important when the field of view is small. Spherical aberration causes those parts of a pencil which fall near the boundary of a convex lens to converge to a nearer (i.e. shorter) focus than those which fall near the centre. This may be corrected by a proper selection of the forms of the two lenses which replace, in all modern telescopes, the single lens hitherto considered.
The false colouring of the image is due to chromatic aberration. The pencil of light proceeding from a point, converges, not to one point, but to a short line of varying colour. Thus a series of coloured images is formed, at different distances from the object-glass. So that, if a screen were placed to receive the mean image in focus, a coloured fringe due to the other images (out of focus, and therefore too large) would surround the mean image.
Newton supposed that it was impossible to get rid of this defect, and therefore turned his attention to the construction of reflectors. But the discovery that the dispersive powers of different glasses are not proportional to their reflective powers, supplied opticians with the means of remedying the defect. Let us clearly understand what is the discovery referred to. If with a glass prism of a certain form we produce a spectrum of the sun, this spectrum will be thrown a certain distance away from the point on which the sun's rays would fall if not interfered with. This distance depends on the refractive power of the glass. The spectrum will have a certain length, depending on the dispersive power of the glass. Now, if we change our prism for another of exactly the same shape, but made of a different kind of glass, we shall find the spectrum thrown to a different spot. If it appeared that the length of the new spectrum was increased or diminished in exactly the same proportion as its distance from the line of the sun's direct light, it would have been hopeless to attempt to remedy chromatic aberration. Newton took it for granted that this was so. But the experiments of Hall and the Dollonds showed that there is no such strict proportionality between the dispersive and refractive powers of different kinds of glass. It accordingly becomes possible to correct the chromatic aberration of one glass by superadding that of another.
This is effected by combining, as shown in [fig. 4], a convex lens of crown glass with a concave lens of flint glass, the convex lens being placed nearest to the object. A little colour still remains, but not enough to interfere seriously with the distinctness of the image.
But even if the image formed by the object-glass were perfect, yet this image, viewed through a single convex lens of short focus placed as in [fig. 1], would appear curved, indistinct, coloured, and also distorted, because viewed by pencils of light which do not pass through the centre of the eye-glass. These effects can be diminished (but not entirely removed together) by using an eye-piece consisting of two lenses instead of a single eye-glass. The two forms of eye-piece most commonly employed are exhibited in [figs. 5] and [6]. [Fig. 5] is Huyghens' eye-piece, called also the negative eye-piece, because a real image is formed behind the field-glass (the lens which lies nearest to the object-glass). [Fig. 6] represents Ramsden's eye-piece, called also the positive eye-piece, because the real image formed by the object-glass lies in front of the field-glass.