OPTICS. 
305 
Baked eye, whatever the size of fhe lens:” 
for the lines FD and FE, if protracted to the 
distance of A and B, would form an image 
exactly twice as large. “ If, oil the other 
hand, the eye is nearer to the lens than the 
focus, it will see the object still larger ; and if 
it is farther than the focus it will not see it so 
large ; and in all cases the visible part of the 
object will be to the lens, as the focal dis- 
tance of the lens is to the distance of the 
eye.” 
From what has been said, the reason will 
be very plain why the magnitude of objects 
seen through a double-convex lens, that is, a 
single microscope, will be in the proportion 
which the focus o£the lens bears to the limits 
of distinct vision. Thus, suppose AB, tig. ], 
to be that distance, or about six inches, so 
that the eye B can but just perceive the ob- 
ject A, and let the focal distance of the lens 
D be one-half of an inch ; then since CD is 
but one-twelfth of'AB, the length of the object 
at C w'ill appear twelve times as large as at 
A, and its surface will appear magnified 1 44 
times. 
The most powerful single microscopes are 
very small globules of glass, which any cu- 
rious person may make for himself by melting 
the ends of line threads of glass in the flame 
of a candle ; or by taking a little fine pow- 
dered. glass on the point of a -very small nee- 
dle, and melting it into a globule in that way. 
It was with such microscopes as these that 
Lewenhoeck made all his wonderful disco- 
veries, most of which are deposited in the Bri- 
tish Museum. 
The double or compound microscope dif- 
fers from the preceding in this respect, that it 
consists of at least two lenses, by one of which 
an image is formed within the tube of the 
microscope ; and this image is view ed through 
the eye-glass, instead of the object itself as in 
the single microscope. In this respect the 
principle is analogous to that of the telescope, 
only that, as the latter is intended to view 
distant objects, the object-lens is of a long fo- 
cus, and consequently of a moderate magni- 
fying power, and the eye-glass of a short fo- 
cus, which magnifies considerably the image 
made by the object lens. Whereas the mi- 
croscope being intended only for minute ob- 
jects, the object-lens is consequently of a 
short focus, and the eye-glass in this case is 
not of so high a magnifying power. 
A single figure will serve to explain the 
principles on which all these instruments are 
constructed. Suppose therefore LN (Plate 
III. fig. 3) to he the object-lens, and FG to 
be the eye-glass. r I he object OB is placed a 
little beyond the principal focus of LN. 'Fhe 
cones or pencils of rays then proceeding from 
the different points of the object, are by the 
lens made to converge to their respective 
foci, and form an inverted image of the ob- 
ject at PQ. This image is seen through the 
eye-glass FG, and the rays of each pencil 
will proceed in a parallel direction to the pu- 
pil of the eye. ^ 
The compound microscope was thus origi- 
nally constructed of two glasses, but it was 
found that what is called the field of view was 
too confined in instruments of this construc- 
tion. For the pencil of rays which emanates 
from the point O of the object, qnd is con- 
verged by the lens to D, would’ proceed af- 
terwards diverging towards II, and therefore 
Vol. II. 
would never arrive at the lens FG, nor enter 
the eye at E ; but the pencils which proceed 
from o and b will be converged to the lens 
EG, and sent to the eye at E in a parallel 
direction. Hence if the object is large, a 
very small part of it will be visible, because 
several pencils will fall without the eye-glass 
FG, and the field of view will consequently be j 
very limited. 
To remedy this inconvenience, a broad 
lens DE is interposed, either of a plano-con- 
vex, or of a double-convex, form. By this, 
it will be perceived, the pencils which would 
have proceeded towards II and I, will be re- 
fracted to the eye-glass, and the figure will 
be completely formed as in the plate. This 
glass is called by opticians the body-glass, be- 
cause it is situated in the body of the mi- 
croscope. Some artists now make these in- 
struments with two eye-glasses, made rather 
thin, which in some degree corrects what is 
called the aberration, or dispersion of the 
rays. In all these microscopes the object is 
seen in an inverted position ; but this is of 
little importance with regard to small insects 
and other minute bodies. 
The solar microscope is a kind of camera 
obscura, which, in a darkened chamber, 
throws the image on a wall or screen. It 
consists of two lenses fixed opposite a hole in 
a board or window-shutter ; one, which con- 
denses the light of the sun upon the object 
(which is placed between them), and the 
other which forms the image. There is also 
a plain reflector placed without, moved by a 
wheel and pinion, which may be so re- 
gulated as to throw the sun’s rays upon 
the outer lens. The reader may form 
some idea of this by inspecting the Plate 
III. fig. 12, of the camera obscura, only 
supposing the figures on the wall to be a 
microscopic object magnified by the lens. 
Mr. Adams’s most ingenious invention, the 
lucernal microscope, is also to be considered 
as a kind of camera obscura ; only the light in 
this latter case proceeds from a lamp, instead 
of from the sun, which renders it convenient 
to be used at all times. But for a descrip- 
tion of this elegant and most amusing instru- 
ment, we must refer to his Microscopical 
Essays. 
From what has been said on the nature of 
the compound microscope, the principle of 
the telescope maybe easily understood. Te- 
lescopes are, however, of two kinds : the one 
depending on the principle of refraction, and 
called the dioptric telescope ; the other on 
the principle of reflection, and therefore 
termed the reflecting telescope. 
The parts essential to a dioptric telescope 
are, the two lenses AD and EY (Plate III. 
fig. 4). As in the compound microscope, 
AD is the object-glass, and EY is the eye- 
glass ; and these glasses are so combined in 
the tube, that the focus F of the one is ex- 
actly coincident with the focus of the other. 
Let OB then represent a very distant ob- 
ject, from every point of which pencils of 
rays will proceed so little diverging to the 
object-lens AD, that they may be considered 
as nearly parallel ; 1M will then be the image 
which would be formed on a screen by the 
action of the lens AD. For supposing OA 
and BD two pencils of rays proceeding from 
the extreme points of the object, they will 
unite in the focal point F, and intersect each 
Qq 
other. But the point F is also the focus of 
the eye-glass EY ; and therefore the pencil 
of rays, instead of going on to diverge, will 
pass through it in nearly a parallel direction, 
so as to cause distinct vision. 
It is then plain that, as in the compound 
microscope, it is the image which is here 
contemplated ; and this will account for the 
common sensation when people say the ob- 
ject is brought nearer by a telescope. For 
the rays, which after crossing proceed in a 
divergent state, fall upon the lens EY, as if 
they proceeded from a real object situated at 
F. All that is effected by a telescope then 
is, to form such an image of a distant object, 
by means of the object-lens, and then to give 
the eye such assistance as is necessary for 
viewing that image as near as possible ; so 
that the angle it shall subtend at the eye shall 
be very large compared with the angle which 
the object itself would subtend in the same 
situation. This is effected by means of the 
eye-glass, which refracts the pencils of rays, 
so that they may be brought to their several 
foci by the humours of the eye, as has been 
described. 
To explain clearly, however, the reason 
why it appears magnified, we must again have 
recourse to the figure. OB being at a great 
distance, the length of the telescope is incon- 
siderable with respect to it. Supposing, 
therefore, the eye viewed it from the centre 
of the object-glass C, it would see it under 
the angle OCB : let OC and BC then be 
produced to the focus of the glass, they will 
then limit the image IM formed in the focus. 
If then two parallel rays are supposed to pro- 
ceed to the eye-glass EY, they will be con- 
verged to its focus Id, and the eye will see 
the image under the angle EHY.' The ap- 
parent magnitude of the object seen by the 
naked eye is, therefore, to that of the ima<*e 
which is seen through the telescope, as the 
magnitude of the angle OCB, or ICM, to 
that of EHY, or 1GM. Now the angle 
IGM is to ICM as CF to FG ; that is, as the 
focal length of the object-glass to that of 
the eye-glass. 
The magnifying power of these glasses 
may be augmented to a considerable degree, 
because the focal length of the object-glass, 
with respect to that of the eye-glass, may be 
greatly increased. This however would re- 
quire a tube of immense length ; because an 
eye-glass of a very short focus would cause 
such a dispersion of the rays of light, parti- 
cularly towards the edges of the glass, that 
the view would be intercepted by the prisma- 
tic colours. 
Another manifest defect in these telescopes- 
is, that the image appears inverted: this, 
however, is of no consequence with respect 
to the heavenly bodies ; and on this account 
it is still used as an astronomical telescope. 
One of almost a similar construction is also 
used on board of ships as a night-glass, to 
discover rocks in the ocean, or an enemy’* 
fleet. Notwithstanding the inconvenience' of 
exhibiting the objects inverted, more glasses 
than two cannot be employed from the pau- 
city of lrglit ; and habit soon enables the per- 
sons who use them to discern objects with to- 
lerable distinctness. 
Galileo, who had heard of the invention 
of telescopes, but had not seen one, con- 
structed a telescope upon theoretical princi- 
