TELESCOPE. 
by the arm, H, directly facing the large 
speculum, B E. Two lenses, w x and n o, 
are contained in the eye tube, and the ob- 
server applies his eye to a small hole at P, 
in order to view a magnified distant object. 
The large reflector, BE, receives the 
rays, ac, bd, from the distant object, and 
reflects them to its focus, e, where they form 
the inverted image, or where they cross 
each other, and then fall divergingly upon 
the small reflector, x y, whose focus is aty ; 
viz. a little further than the focus, e, of the 
large reflector : hence the rays are reflected 
back upon the lens, to x, not in a parallel, 
but in a converging manner ; and that con- 
vergency is increased by the action ot that 
lens, so as to come to a focus, or to form a 
second image, R S, much larger than the 
former, and erect like the object. Lastly, 
this image is viewed through the eye lens, 
n 0 ; or, in other words, the rays from every 
single point of the object, after this second 
crossing, fall divergingly upon the eye lenli, 
which sends them nearly parallel to the eye 
at P, through a very small hole. Some- 
times the eye lens, n o, is double, rAz, it 
consists of two lenses, which perform the 
office of a single lens. 
If the first lens, w x, were removed, the 
image would.be formed some,what larger 
at z; but the area or field of view would be 
smaller and less pleasant. At the place of 
the image, R S, there is situated a circular 
piece of brass, called a diaphragm, with a 
hole of a proper size to circumscribe the 
image, and to cut off all superfluous or ex- 
traneous light, in order that the object may 
appear as distinct as possible. 
The magnifying power of this telescope 
is computed in the following manner: If 
this telescope consisted of the two reflec- 
tors only, and these were situated so that e 
were the focus of each reflector ; then the 
rays which came parallel from the distant ob- 
ject to the large reflector, and divergingly 
t'rom that to the small reflector, would, after 
the second reflection, go parallel to the eye 
at P, and of course the object would ap- 
pear magnified in the proportion of the fo- 
cal distance of the large reflector to the 
foeal distance of the small reflector; so 
that if the focal distance of the former be 
to that of the latter as six to one, then the 
object would be magnified six times in dia- 
meter. But since the first image is magni- 
fied into a second image much larger, which 
is viewed through the eye lens ; therefore 
the whole magnifying power is in a propor- 
tion compounded of d e to ex, and of s x 
to z 0. If the former proj^orlion be as six 
to one, and the latter as eight to one ; then 
the object will appear forty-eight {viz. six 
by eight) times larger in diameter through 
the telescope than to the naked eye. 
The fourth species of reflecting telescope 
goes under the name of “ Cassegrainian Te- 
lescope.’’ It diflfers from the preceding, in 
having the small reflector convex, instead 
of concave; in consequence of which the 
small reflector must be placed nearer to the 
large reflector than the focus of the latter; 
then the rays .from the large reflector fall 
convergingly upon the convex small reflec- 
tor, and are by it sent back' convergingly 
upon the lens, w x, &e, Tire chief dif- 
ference between this and the preceding 
telescope is, that in this the object appears 
inverted, because in it there is no image 
formed, or the rays do not cross each other, 
between the two reflectors. Also with the 
same magnifying pow'er, &c. this telescope 
is shorter than the Gregorian, by twice the 
focal length of the small speculum. 
To both those telescopes a long wire is 
fixed all along the outside of the tube, at 
the end of which there is a screw which 
works into an external projection, g, of the 
internal arm, H, and serves to move that 
arm with the small speculum nearer to or 
further from the large speculum, in order to 
adjust the focus of the instrument, accord- 
ing to the distance of the object. The ac- 
tion of this wire is easily understood ; for it 
passes through a hole at F, where it is pre- 
vented going forwards or backwards by 
two shoulders, which are indicated by the 
figure: hence, when the observer looks 
through the hole, P, he turns with his hand 
the wire by the nut, Q, which screws the 
projection, g-, of the arm nearer or further, 
&c. until the object appears very distinct. 
The largest reflecting telescope now ex- 
isting, was constructed by that excellent 
astronomer. Dr. Herschel. It is a tele- 
scope in which the observer looks through 
an eye lens down upon the large reflector, 
whose polished surface is forty-eight inches 
in diameter. Its focal length is about forty 
feet. 
There are however two useful appen- 
dages to telescopes, which deserve to be 
briefly described. A finder, viz. a short 
telescope A, fig. 3 , is generally affixed to 
the tube of a large telescope, for the pur- 
pose of finding out an object expeditiously. 
This finder does not magnify the object 
more than four, six, or eight times ; but it 
Las a great field of view, so that through if 
