[Illustration: FIG. 15.—Diagram of Huygens’ Eyepiece.] This is shown in section in Fig. 15. It consists of a field lens _A_, plano-convex, and an eye lens _B_ of one-third the focal length, the two being placed at the difference of their focal lengths apart with (in later days) a stop half way between them. The eye piece is pushed inside the main focus until the rays which fall on the field lens focus through the eye lens. The great gain from Huygens’ view-point was a very much enlarged clear field—about a four-fold increase—and in fact the combination is substantially achromatic, particularly important now when high power oculars are needed. Still larger progress was made in giving the objective a better form with respect to spherical aberration, the “crossed” lens being rather generally adopted. This form is double convex, and if of ordinary glass, with the rear radius six times the front radius, and gives even better results than a plano-convex in its best position-plane side to the rear. Objectives were rated on focal length for the green rays, that is, the bright central part of the spectrum, the violet rays of course falling short and the red running beyond. To give customary dimensions, a telescope of 3 inches aperture, with magnifying power of 100, would be of about 30 feet focus with the violet nearly 6 inches short and the red a similar amount long. It is vast credit to the early observers that with such slender means they did so much. But in fact the long telescope had reached a mechanical _impasse_, so that the last quarter of the seventeenth century and the first quarter of the next were marked chiefly by the development of astronomy of position with instruments of modest dimensions. [Illustration: FIG. 16.—The First Reflector. John Hadley, 1722.] In due time the new order came and with astounding suddenness. Just at the end of 1722 James Bradley (1692-1762) measured the diameter of Venus with an objective of 212 ft. 3 in. focal length; about three months later John Hadley (1682-1744) presented to the Royal Society the first reflecting telescope worthy the name, and the old order practically ended. John Hadley should in fact be regarded as the real inventor of the reflector in quite the same sense that Mr. Edison has been held, _de jure_ and _de facto_, the inventor of the incandescent electric lamp. Actually Hadley’s case is the stronger of the two, for the only things which could have been cited against him were abandoned experiments fifty years old. Moreover he took successfully the essential step at which Gregory and Newton had stumbled or turned back—parabolizing his speculum.