HOW TO WORK WITH THE TELESCOPE. 
283 
the Galilean telescope is not an illuminating telescope,* and the 
extent of its field of view depends on the aperture of the object- 
glass, not of the eye-glass. We shall presently see that 
these points are highly important. 
In considering the application of the telescope to practical 
observation, the circumstance that in the Galilean telescope 
no real image is formed, is yet more important, since a real 
image admits of measurement, linear or angular, while to a 
virtual image (such an image, for instance, as is formed by a 
common looking-glass), no such process can be applied. In 
simple observation the difference referred to is not of much 
importance perhaps, the only noticeable effect being that, where- 
as, in the astronomical telescope, a stop or diaphragm can be in- 
serted in the tube so as to cut off what is called the ragged edge of 
the field of view, there is no means of remedying the corre- 
sponding defect in the case of the Galilean telescope. It would 
be a very annoying defect in a telescope intended for astro- 
nomical observation, since in general the edge of the field of 
view is not perceptible at night. The unpleasant nature of 
the defect may be seen by looking through an opera-glass, 
and noticing the gradual fading away of fight round the cir- 
cumference of the field of view. 
The properties of reflection as well as of refraction have 
been enlisted into the service of the astronomical observer. 
The formation of an image by means of a concave mirror is 
exhibited at fig. 3. As the observers head would be placed 
between the object and the mirror, if the image, formed as in 
fig. 3, were to be microscopically examined, various devices are 
employed in the construction of reflecting telescopes to avoid 
the loss of light which would result, a loss which would be 
important even with the largest mirrors yet constructed. 
Thus in Gregory's telescope a small mirror, having its con- 
cavity towards the great one, is placed in the axis of the 
tube, and forms an image which is viewed through an aperture 
in the middle of the great mirror ; a like plan is adopted in 
Cassegrain's telescope, a small convex mirror replacing the 
small concave one. In Newton's telescope an inclined -plane 
reflector is used, which sends the pencil of light off at right 
angles to the axis of the tube ; and in Herschel's telescopef 
the great mirror is inclined so that the image is formed at a 
* C D is larger than the aperture of the eye, but in a ratio which is almost 
exactly equivalent to the magnifying power of the telescope, so that the 
apparent brightness of an object remains unchanged when it is viewed through 
a Galilean telescope. 
f Sometimes called LeMaire’s telescope. In reality very little credit can be 
claimed for the invention of new forms of the reflecting telescope. “ I doubt 
VOL. V. — NO. sj:. X 
