The Achromatic Refractor

Dollond was making refractors (spyglasses) with single-lens objectives as early as 1742, his price for a 2-foot telescope then being 7s 6d.* In comparison, in 1762 he sold a 2-foot telescope with a two-lens objective (achromat) for 2 guineas. The lens diameters in each case were just under 2".

In 1783, with a view to combining the benefits of the wide field of Huygens' eyepiece with a means of making micrometric measurements of an image in the focal plane, Jesse Ramsden, an English optician, designed the compound eyepiece shown in Fig. 68. ft can be seen (more clearly, perhaps, from Fig. 65) that a

*In English currency, 12 pence is equal to 1 shilling; 20 shillings, 1 pound. A shilling is normally equivalent to 24 cents.

measuring device, such as adjustable parallel wires, set in the focal plane would be magnified along with the image. Measurement of an image in the focal plane was by no means a new idea; probably this had been first accomplished by Gascoigne, an Englishman, about 1638.

With the advent of the achromatic lens, the erecting or terrestrial eyepiece assumed considerable importance. This type of eyepiece was first suggested and used by Kepler, and improved in design about 1645 by Antonios Maria v. Schyrle, a Capuchin monk better known as Rheita. It is mentioned here because it spelled the rise of the refractor and the decline of the Gregorian for
terrestrial use.

 As the terrestrial eyepiece has no significance in astronomical instruments, it will not be further discussed. Information on it can be found in any treatise on optical instruments. In the early part of the 1.9th century, small achromatic re-
fractors were being manufactured by several concerns. Prices, for apertures of 2" to 3l/2", ranged from 10£ to 50£. This included a pillar and claw stand, and in most cases, a rack work for focusing.

With the larger sizes, three eyepieces of different powers were also included. A 5½-inch refractor by Dollond sold for 350 guineas. The lens alone could be purchased for 200 guineas. For those not having the means to buy achromats, telescopes with single-lens objectives continued to be made. Enterprising opticians were also offering lens sets that could be assembled into simple refractors.

The Modern Era