A highly specialized form of triplet is the so-called monocentric of Steinheil Fig. 99_b_. Its peculiarity is less in the fact that all the curves are struck from the same center than in the great thickness of the front flint and the crown, which, as in some photographic lenses, give added facilities for flattening the field and eliminating distortion. 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