306 
pies, and adopted a concave lens as an eye- 
glass, but whether with a view of obviating 
the disagreeable effect produced by the in- 
version of the image or not is uncertain. 
This effect is however produced by the Gali- 
lean telescope, the construction of which is 
as follows : Let AB, fig. 5, be a very dis- 
tant object, from every point of which pencils 
of rays proceed to the convex lens DR, and 
are refracted towards their foci at FSG. But 
a concave lens HI, the virtual focus of which 
is at FG, being interposed, the rays are not 
suffered to converge to that point ; but being 
made less convergent, as is the effect of these 
glasses, enter the pupil almost parallel, and 
are converged by the humours of the eye to 
their proper foci on the retina at FQR : and 
the object will appear erect, because the 
pencils of rays cross each other only once, 
as in natural vision. Objects are seen very 
distinct through this telescope ; but the field 
of view is so small, tiiat its use is almost ex- 
clusively confined to the common opera- 
glasses. For if the focus of the eye-glass is 
short, the pencils of rays are rendered so di- 
vergent, that but a few 7 of them can enter the 
pupil. 
it was necessary then, to render the diop- 
tric telescope useful for terrestrial purposes, 
to cause the image to be seen in an erect po- 
sition. This was effected by the addition of 
two other convex lenses; of this Kepler sug- 
gested the idea, though it was not reduced to 
practice till thirty years after his time. The 
principle on which this telescope is construct- 
ed will be easily understood from what has 
been premised, and by inspecting the Plate, 
tig. 6. It will be seen there, that to the com- 
mon astronomical telescope, there are added 
two other eye-glasses of the same focus as 
the first, L'vl and QR ; and the first of these 
is placed at twice its focal distance from III. 
Alter the rays therefore have passed the first 
eye-glass HI, instead of being received by 
the eye, as in the former case of the astrono- 
mical telescope, they pass on; the rays which 
constitute each pencil being rendered paral- 
lel : and in this state the respective pencils 
cross each other in the common focus, and 
the rays are received in this parallel state by 
the second eye-glass LM. The rays then 
constituting the respective pencils converge 
lo their foci at NO, where a second image is 
formed, but inverted with respect to the 
former image EF. This then is the image 
which is viewed through the third eye-glass 
QR ; and being in the same position as the 
object itself, is painted on the retina at XZY, 
and causes the object to be seen erect, as if 
no glasses had been interposed. The appa- 
rent magnitude of the object is not changed 
by these glasses ; and depends, as before, on 
the focal lengths of the hrst object-glass and 
the lens nearest to it. The brilliancy of the 
object, however, will be diminished, since 
several rays will be lost in their passage 
through the two additional glasses. In placing 
the glasses in this telescope, care must be 
taken that the axes of the lenses coincide, or, 
as it is evident from our principles, indistinct 
vision only will be produced. 
The brightness of the appearance through 
any of these telescopes or microscopes, de- 
pends chiefly on the aperture of the object- 
glass. For if the whole of that glass was 
covered except a small aperture in the mid- 
dle, the magnitude of the image would not 
OPTICS. 
be altered ; but fewer rays of every pencil 
being admitted, the object would appear ob- 
scure. 
In few words, the apparent distinctness or 
confusion of any object, viewed through 
glasses, depends bn the mutual inclination of 
the rays in any one pencil to each other, 
when they fall on the eye; the. apparent 
magnitude depends upon the inclination of 
the rays of different pencils to each other ; 
the apparent situation depends upon the real 
situation of the extreme pencils ; and the ap- 
parent brightness or obscurity depends on 
the quantity of rays in each pencil. 
As the magnifying power of all dioptric te- 
lescopes depends on the proportion which 
the focal length of the eye-glass bears to that 
of the object-glass ; and as an eye-glass of 
very high magnifying powers ctuld not be 
used on account of the aberration or disper- 
sion of the rays, from the unequal thickness 
of the glass ; various contrivances were in- 
vented for tlie sake of employing object- 
glasses of a very long focus. Wooden tubes 
of a very great length were found unma- 
nageable.' At length the famous Huygens 
invented a mode of dispensing with the tube. 
He attached the object-glass to a high pole, 
with a piece of mechanism which enabled 
him to raise or lower it at pleasure ; and he 
made the eye-glass correspond to it by a silk 
cord, which he held tight in his hand. This 
method is, we believe, still in use on the 
continent for celestial objects, and distin- 
guished by the name of the aerial telescope. 
These inventions were however all render- 
ed nugatory by the discovery of the reflect- 
ing telescope. For a dioptric or refracting 
telescope, even of one thousand feet focus, if 
it could be used, could not be made to mag- 
nify with distinctness above one thousand 
times ; whereas a reflecting telescope of the 
length of eight or nine feet will magnify with 
distinctness 1200 times. 
The well-known property in concave spe- 
culums, of causing the pencils of rays to con- 
verge to their foci, and there forming an 
image of any object that may be opposed to 
them, gave rise to the reflecting telescope. 
In this the effect is precisely the same as that 
produced by the dioptric telescope; only 
that in the one case it is produced by reflect- 
ed, and in the other by refracted, light. Re- 
flecting telescopes are made in various forms ; 
and those principally in use in this country 
are distinguished by the names of their re- 
spective inventors, and are called the New- 
tonian, Gregorian, and Hersehelian tele- 
scopes. The reflecting telescope on the Gre- 
gorian principle, which is the most common, 
as it is found to be the most convenient, is 
consructed in the following manner : 
At the bottom of the great tube (Plate 
HI. fig. 7) TTTT, is placed a large 
concave mirror DUVF, whose principal fo- 
cus is at rn: and in the middle of this mirror is 
a round hole P, opposite to which is placed 
the small mirror L, concave toward the great 
one ; and so fixed to a strong wire M, that it 
may be removed further from the great mir- 
ror, or nearer to it, by means of along screw 
in the inside of the tube, keeping its axis still 
in the same line P mn with that of the great 
one. Now, since in viewing a very remote 
object, we oan scarcely see a point of it but 
what is, at least, as broad as the great mii- 
ror, we may consider the rays of each pen- 
cil, which now from every point of the ob- 
ject, to be parallel to each other, and to co- 
ver the whole reflecting surface DUVF. But 
to avoid confusion in the figure, we shall only 
draw two rays of a pencil flowing from each 
extremity of the object into the great tube ; 
and trace their progress through all their re- 
flections and retractions to the eye j at the 
end of the small tube tt, which is joined to 
tlie great one. 
Let us then suppose the object AB to be at 
such a distance, that the rays C may flow 
from its upper extremity A, and the rays E. 
from its lower extremity B; then the rays C 
falling parallel upon the great mirror at D, 
will be thence reflected converging in the, 
direction DG ; and by crossing at I in the 
principal focus in the mirror, they will form 
the lower extremity of the inverted image 
IK, similar to the upper extremity A of the 
object AB ; and passing on to the concave 
mirror L (whose focus is at n), they will fall 
upon it at g, and be thence reflected, con- 
verging in the direction gN, because gm is- 
longer than gn ; and passing through the hole 
P in the large mirror, they would meet some- 
where about r, and form the upper extre- 
mity a of the erect image ab, similar to the 
i upper extremity A of the object AB. But by 
I passing through the plano-convex glass R ia 
their way, they form that extremity of the 
image at a. In the same manner the rays E, 
which come from the bottom of the object 
AB, and fall parallel upon the great mirror 
at F, are thence reflected, converging to its 
focus ; where they form the upper extremity 
I of the inverted image IK, similar to the 
lower extremity 13 of the object AB : and 
thence passing on to the small mirror L, and 
falling upon it at h, they are thence reflected 
in the converging state AO ; and going on 
through the hole P of tlie great mirror, they 
would meet somewhere about q ,. and form 
there the lower extremity b of the erect 
image ab, similar to the lower extremity ft 
of the object AB ; but by passing through, 
the convex glass R in their way, they meet 
and cross sooner, as at b, where that point of 
the erect image is formed. The like being 
understood of all those rays which flow from 
the intermediate points of the object between 
A and B, and enter the tube TT, all the in- 
termediate points of the image between a and 
b will be formed ; and the rays passing on 
from the image- through the eye-glass S, and 
through a small hole e in the end' of the lesser’ 
tube tt, they enter the e\ e j\ which sees the 
image ab (by means of the eye-glass) under 
the large angle ced, and magnified in length 
under that angle from c to d. 
In tlie best reflecting telescopes, the focus 
of the small mirror is never coincident with 
the focus m of the great one, where the first 
image IK is formed, but a little beyond it 
(with respect to the eye) as a fen; the conse- 
quence of which is, that the rays of the pen- 
cils will not be parallel after reflection from 
the small mirror, but converge so as to meet 
in points about q, c, r ; where they would 
form a larger upright image than ab, if the 
glass R was not in their way, and this image 
might be viewed by means of a single eye- 
glass properly placed between the image and 
the eye : but then the field of view would be 
less, and consequently not so pleasant ; for 
