Modern speculum metal is substantially a definite compound of four atoms copper and one tin (SnCu4), practically 68 per cent copper and 32 per cent tin, and is now, as it was in all previous modifications, a peculiarly mean material to cast and work. Thus exit the reflector.
Fig. 15.—Diagram of Huygens’ Eyepiece.
The long telescope continued to grow longer with only slow improvement in quality, but the next decade was marked by the introduction of Huygens’ eyepiece, an immense improvement over the single lens which had gone before, and with slight modifications in use today.
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.
Fig. 16.—The First Reflector. John Hadley, 1722.