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POPULAR SCIENCE REVIEW. 
Suppose, for instance, that those rays of light proceeding from 
the point P (slightly removed from the line of the telescope’s 
axis), which fall upon the object-glass, converge so as to form 
the point p of the image, then after such convergence they 
are received as a diverging pencil upon a part of the eye-glass, 
and (supposing this glass properly adjusted for distinct vision) 
they emerge as parallel rays. It will be seen that the central 
ray of the pencil falls on the centre of the object-glass, but not 
upon the centre of the eye-glass ; this is technically expressed 
by saying that the pencil is refracted centrically through the 
object-glass and excentrically through the eye-glass. It follows 
that the whole of the pencil gdes to the formation of the image 
of the point P, so long as the whole of the pencil falls on the 
eye-glass ; that if any part of the object-glass is covered in 
any way the point p of the image will still be visible, its illu- 
mination being, however, diminished ; but that if that part of 
the eye-glass on which the pencil falls is covered, the point p 
will not be visible. In other words, the astronomical refractor 
is an illuminating telescope , its illuminating power depends 
ivholly on the size of the object-glass ,* and the extent of the field 
of view depends ivholly on the size of the eye-glass. 
The effect of these points will be more readily appreciated 
if we compare the astronomical with the Galilean refractor, 
exhibited at fig. 2. Surprise is sometimes expressed that 
this instrument, which in the hands of Galileo effected such im- 
portant astronomical discoveries, should now be known as the 
non- astronomical telescope ; but this will be readily understood 
when we compare the two arrangements. The oblique pencil 
from P fig. 2, which, after refraction through the lens A B, 
would proceed to convergence at p, as in the former case, is 
in the Galilean telescope intercepted by the concave lens a b 
(so placed that its distance from A B equals the difference of 
the focal lengths of the two lenses), and being formed of 
parallel rays after emergence affords distinct vision of the 
point P. Now only that part of the pencil which falls be- 
tween the points C D of the object-glass is thus intercepted by 
the eye-glass, so that if this part of the object-glass were 
covered, the point P would be invisible, but a part of the eye- 
glass might be covered without rendering P invisible. Thus 
* This seems to be contradicted by the circumstance that the higher the 
magnifying power of the eye-glass, the smaller is the apparent illumination of 
the image ; it is strictly true, nevertheless, — since, whatever the power of the 
eye-glass, the absolute quantity of light received from each point of a luminous 
image remains constant. 
