64 
THE YOUNa SCIENTIST. 
achromatic, but with loss of light. The 
chief disadvantage of reflectors is the 
greater aperture, and consequently greater 
atmosplieric disturbance, corresponding 
with the same amount of light ; and the 
occasional renewal of the tilni causes a 
little expense or trouble. 
The eye-piece, or ocular, is only a kind 
of microscope, magnifying the image 
formed in the focus of the object-glass or 
speculum. The size of this image being 
in proportion to its distance from the 
glass or mirror which forms it, the power 
of the same eye- piece in different tele- 
scopes varies as the focal length. Hence 
the disadvantage of a short telescope ; to 
get high powers, we must employ minute 
and deeply-curved lenses, which are much 
less pleasant in use ; with a telescope twice 
as long, half the curvature in the eye-piece 
produces an equal power. The magnified 
focal image, as in the camera, is always in- 
verted, and so in the astronomical eye- 
piece it remains.* For terrestrial purposes 
it is erected by two additional lenses ; but 
a loss of light is thus incurred, and as the 
inversion of celestial objects is unimpor- 
tant, erecting eye-pieces (always the 
longest of a set) should never be em- 
ployed for astronomy; the eye soon be- 
comes accustomed to the inverted picture, 
and the hand to the reversed motion in 
following the object. The lateral vision 
in the Newtown ian reflector interposes 
another difficulty, easily mastered, how- 
ever, by practice, and by attention to the 
direction of motion through the field. A 
multitude of eye-pieces is needless, but 
three at least are desirable ; one with low 
power and large field, for extended groups 
of stars, nebulae, and comets, supplying 
also, if necessary, the place of a "finder" 
lor deep magnifiers; a stronger one for 
general purposes, especially the moon 
and planets ; and a third, as powerful as 
the telescope will bear, for minute objects, 
especially double stars. A greater num- 
ber of eye-pieces admits, however, of what 
is often important — an adaptation of the 
power to the brightness of the object. 
*It is erect in the Galilean eye-piece and the 
Gregorian reflector. But the use of the former is 
almost confined to opera-glasses, as its field with 
high powers is exceedingly small : and the latter, 
an inferior construction, is now little employed. 
Ordinary astronomical eye-pieces are | 
shorter in proportion to their power. 
It is a better plan to change them by 
means of a short tube, or "adapter," | 
than by a screw ; in which case they are \ 
more liable to be dropped and injured. ' 
The power may be much increased by 
unscrewing and and taking away the 
" field-lens " — that farthest from the eye; 
but the centre of the field only will be 
distinct. The highest powers of large 
telescopes are sometimes made thus, with 
single lenses for the advantage of light ; 
but the lens is then turned the other way, 
convex towards the eye, as it gives sharper 
vision. Sir W. Herschel used the double 
convex form, as having shallower curves. 
The common kind, with two lenses, hav- 
ing the flat side of each next the eye, is 
called the Huyghenian or negative eye- 
piece : the positive or Eamsden eye-piece 
has a flatter field, but is not, like the other, 
achromatic. The interposition of a com- 
bination called a Barlow lens raises the 
power with little loss of light ; and as one 
may be made to suit all the eye-pieces, it 
doubles the set at a small expense. Brown- 
ings's achromatic eye-piece, and Horne 
and Thornthwaite's aplanatic, and the 
Kellner construction (for large fields) are 
all excellent in their way. The brightness 
of the field varies inversely as the square 
of the power : and hence minute stars are 
commonly more visible with deep eye- 
pieces; the reverse, however, for some 
unknown reason, sometimes occurs. 
If the power of our oculars has not been 
engraved upon them,* we may get a fair 
approximation to it by viewing an equally 
divided scale at a distance (for low powers, 
a brick wall will answer) with one eye 
through the telescope, and with the other 
alongside of it, and noting how many un- 
magnified divisions are covered by a single 
magnified image. Or, better still, we may 
have recourse to the Berthon Power- 
gauge, a little apparatus, the simple, ef- 
ficient, and inexpensive character of which, 
entitles it to very warm comendation. 
* These figures are not, however, always to be 
depended upon, and must be wrong if the eye- 
piece was made for an instrument of a different 
focal length. The celebrated Short exaggerated 
the powers of his reflectors ; and those of the 
great achromatics of Dorpat and Berlin were 
found by Struve and Eneke to be overrated. 
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