156 ASTRONOMY. 
obtains (as also happens in astronomical telescopes) a second eye-glass: 
viz. the so called field glass, for the purpose of enlarging the field of 
vision. . 
The Galilean telescope is the simplest of all, consisting of but two lenses— 
a convex object glass, and a concave eye-glass; this represents objects 
erect. It has, however, the inconvenience of possessing a very small — 
field of vision, especially when the eye is not exceedingly near the eye- 
glass. 
The common telescopes, however, do not exhibit any very great degree 
of clearness of vision, as all images produced by them are surrounded by 
a colored border, and a considerable magnifying power can only be obtained 
with a length of 5 or 6 feet. Dollond remedied this defect in 1757 by his 
achromatic (colorless) object glasses. An achromatic object glass consists 
of two highly polished lenses, combined so as to be everywhere in absolute 
contact, the one convex, made of crown glass, the other concave, of flint 
glass. The two kinds of glass have different refractive powers, so that 
chromatic aberration is reduced almost to a minimum. ‘Telescopes with 
such object glasses, and of a moderate length, afford with a considerable 
magnifying power, and great brightness and distinctness, images almost 
entirely without colored borders. Very large instruments of this kind are 
known pre-eminently as refractors. The preparation of the glass required, 
so as to be everywhere uniform and without bubbles and streaks, pre- 
sented great difficulties, until they were surmounted by Fraunhofer and 
Uzschneider. 
On account of the differences in eyes, and the varying distances of 
objects, a special adjustment is required for the eye-piece. In small tele- 
scopes this is done by moving the tube in or out with the hand alone; in 
larger instruments a screw adjustment is necessary, by which the eye-piece 
(ocular) can be moved almost insensibly. The larger telescopes are set up 
either on a three-legged frame or upon a pyramidal support, and their 
motions are either horizontal and vertical, or for astronomical purposes 
parallactic, 1. e. applicable to any direction. 
As an illustration of the mode of constructing and setting up a great tele- 
scope, we have selected the giant achromatic refractor constructed by 
Fraunhofer for the observatory of Dorpat. The instrument arrived there 
November 10th, 1824, and the first glance was directed by Struve, six 
days afterwards, towards the moon and some double stars. The principal 
parts of the instrument (pl. 15, fig. 2) are—l, the stand A, A, A; 2, the axes 
F and I, with their circles d and k; 3, the telescope B, B; 4, the counter- 
poises KH, E’, K, M, H; 5, the clock-work, e, f, g, with the weights. The 
stand is parallactic ; upon it rests the hour axis F, parallel to the polar axis, 
with its hour circle, d, at the lower end. A second axis, I, stands perpen- 
dicular to the first, and consequently in a rotation of the instrument about 
the latter, describes the plane of the equator. At one end of this second 
axis is the declination circle, k ; at the other, the bed of the wooden tube, B, B, 
of the telescope, with brass caps at the two ends for the lenses, and the 
finder, D, D, at the upper side. The counterpoises are five ; two, K and E’ 
156 
