The monocentric eyepiece has a high reputation for keen definition and is admirably achromatic and orthoscopic. The sharp field is about 32°, rather the largest given by any of the cemented combinations. All these optically single lenses are quite free of ghosts, reduce scattered light to a minimum, and leave little to be desired in precise definition. The weak point of the whole tribe is the small field, which, despite Herschel’s opinion, is a real disadvantage for certain kinds of work and wastes the observer’s time unless his facilities for close setting are more than usually good.
Hence the general use of oculars of the two lens types, all of them giving relatively wide fields, some of them faultless also in definition and orthoscopy. The earliest form, Fig. 100, is the very useful and common one used by Huygens and bearing his name, though perhaps independently devised by Campani of Rome. Probably four out of five astronomical eyepieces belong to this class.
The Huygenian ocular accomplishes two useful results—first, it gives a wider sharp field than any single lens, and second it compensates the chromatic aberration, which otherwise must be removed by a composite lens. It usually consists of a plano-convex lens, convex side toward the objective, which is brought inside the objective focus and forms an image in the plane of a rear diaphragm, and a similar eye lens of shorter focus by which this image is examined.
Fig. 100 shows the course of the rays—A being the field lens, B the diaphragm and C the eye lens. Let 1, 2, be rays which are incident near the margin of A. Each, in passing through the lens, is dispersed, the blue being more refracted than the red. Both rays come to a general focus at B, and, crossing, diverge slightly towards C.
But, on reaching C, ray 1, that was nearer the margin and the more refracted because in a zone of greater pitch, now falls on C the nearer its center, and is less refracted than ray 2 which strikes C nearer the rim. If the curvatures of A and C are properly related 1 and 2 emerge from C parallel to each other and thus unite in forming a distinct image.
Now follow through the two branches of l marked l_r, and l_v, the red and violet components. Ray l_v, the more refrangible, strikes C nearer the center, and is the less refracted, emerging from C substantially parallel with its mate l_r, hence blending the red and violet images, if, being of the same glass, A and C have suitably related focal lengths and separation.
As a matter of fact the condition for this chromatic compensation is
d = (f + f′)/2
where d is the distance between the lenses and f, f′, their respective focal lengths. If this condition of achromatism be combined with that of equal refraction at A and C, favorable to minimizing the spherical aberration, we find f = 3f′ and d = 2f′. This is the conventional Huygenian ocular with an eye lens ⅓ the focus of the field lens, spaced at double the focus of the eye lens, with the diaphragm midway.
