THE TELESCOPE. 
471 
scrupulously clean ; for this purpose a piece of old lawn is much 
preferable to silk, and some say a well-crumpled piece of tissue- 
paper to either. When the instrument comes from a good 
maker, the centering of the object-glass and its proper 
position in its cell has doubtless been duly taken care of, and 
should not be disturbed. Sometimes, however, the glasses of 
both the object- and eye-lenses are pinched too tightly, which 
distorts the images fearfully ; a little unscrewing will at once 
set this to rights. 
The ordinary achromatic refractor is composed of simply 
the object- and eye-lens, without any intermediate glass. 
This latter, however, like the former, consists generally of 
two separate lenses, although both are of the same sort of 
glass. The negative eye-piece consists of two plano-convex 
lenses, the plane sides being turned towards the object-glass, 
and when made of crown glass, separated by a distance equal 
to half the sum of their focal lengths. When measuring 
apparatus is attached to the telescope, an eye-piece is employed, 
formed of two plano-convex lenses of equal focus, separated 
by a distance equal to two-thirds of the focal length of one or 
other and their convexities opposed in the tube. This is called 
Rams den’s eye-piece, and gives a flat field of view, with the 
advantage already mentioned that a system of spider lines, or 
cross wires, can be distinctly seen at the same time as the double 
star, or planet, whose distance or diameter is to be measured. 
Both of those are inverting, but this is of small account in 
looking at the heavenly bodies. The erecting eye-piece, con- 
sisting of four lenses, although admirably adapted for land use, 
is never made use of for the latter purpose, as each lens tends 
to obstruct the light which is so precious when gathered to a 
focus. It need scarcely be added that the eye-piece must be 
adjusted with the greatest possible nicety for different sights. 
This is easily done by the rack and pinion motion after a few 
trials, whether upon the edge of a planet or by judging of the 
appearance of a double star, or by the visibility of the faintest 
single stars in the field of view. 
With an instrument of three to four inches aperture (and a 
good one of the latter size will bear a magnifying power of 
from 300 to 350) the belts and markings on Jupiter, the snows 
of Mars, the ring of Saturn, may be well seen ; and the spots 
and faculae of the sun, and all the gorgeous phenomena of the 
moon, rendered most perfectly. The nebula of Orion sur- 
rounding the trapezium mil likewise amply repay curiosity on 
clear dark nights. Some hundreds of double stars — shining- 
in all their different lustres, colours, and distances — are a 
constant sorn-ce of pleasure. In addition to these, many 
clusters and nebulae may be seen with such an instrument. 
