OPTICS. 
reach the retina before they are collected 
into a focus ; in this case the defect is sup- 
plied by a concave glass, a 6, which makes the 
point whence the rays diverge, nearer than 
the object ; consequently, the rays falling 
on the eye will now diverge more than be- 
fore, so as when refracted through the hu- 
mours not to come to their focus before 
they reach the retina. 
Spectacles are constructed on the above 
principles, concave for short-sighted, and 
convex for long sighted people. See Spec- 
tacles and Vision. 
“ Of microscopes and other optical in- 
struments.” The impediments to the vision 
of very near objects arise from too great a 
divergence of the rays in each pencil inci- 
dent on the eye, and are remedied by the 
microscope. This instrument is of two 
kinds : 1, refracting ; and, 2, reflecting. 
The refracting microscope is either sin- 
gle or compound. The former is a small 
double convex lens, of a short focal length ; 
the object is placed in its focus, by which 
disposition the rays of each pencil emerg- 
ing from the lens become parallel, and 
so are brought to their respective foci 
on the retina by the humours of the eye : 
the magnifying power of the instrument 
appears from hence. 
The apparent lineal magnitude of an ob- 
ject seen with this instrument, is to its lineal 
magnitude seen with tlie naked eye, as the 
least distance that admits of distinct vision 
with the naked eye, to the focal length of 
the lens ;■ for these magnitmips are as the 
angles under which the object appears, i. e. 
inversely as tlie distances at which it is 
viewed. 
A compound microscope is composed of 
two double convex glasses, the broader 
next the eye. In this instrument the dis- 
tance of the object from the object-glass is 
to be made greater than the focal length of 
tliat lens ; then the image will be formed 
at the focus conjugate to the place of the 
object, and the eye-glass being placed at its 
own focal distance from the image, will 
make the rays emerge parallel to each other, 
and consequently produce distinct vision. 
See Microscope. 
To enlarge the field of the compound 
microscope, it is usual to insert a broad lens, 
as in the astronomical telescope, between 
the object-glass and the image. 
The reflecting microscope is thus con- 
structed : In the extremity of a broad tube 
insert a concave speculum N U (fig. 21) ; 
a point O in its axis, whose distance from 
the vertex, V, is greater than the focal length 
of the concave, is the place for the object, 
whose image will consequently be formed 
at the focus, G, conjugate to the point O : at 
the distance of its own focal length, L Q, 
place a double convex lens, a b, by which the 
image will be seen distinctly. The object 
is illuminated by light admitted into the 
tube through a space, P R, adjoining to the 
speculum ; and the illustration of the object 
may be rendered more intense by a con- 
cave speculum, A B, which shall reflect the 
light so admitted to a focus at the place of 
the object. 
A solar microscope is constructed in the 
following manner ; In the inside of a tube 
is placed a convex lens, A B (fig. 22) ; and 
at a distance a little greater than its focal 
length, but less than double of it, is fixed 
some transparent coloured object, Q P, 
whose image will be painted much enlarged 
at the focus conjugate to the place of the 
object. A broad lens C D, is placed before 
the object to collect the solar rays, for the 
purpose of illuminating it more strongly, and 
consequently making theimage more distinct 
and vivid. On the same principle is con- 
structed the Magic Lantern, which see. 
The camera obsciira is an instrument 
used to facilitate the delineation of pros- 
pects. It is constructed in the following 
manner : A C (fig. 23), represents a box of 
about a foot and a half square, shut on 
every side, except DC; O P is a smaller 
box placed on the top of the greater; 
M N is a double convex lens, whose axis 
makes an angle of 45° with B L, a plane 
niii ror fixed in the box O P ; the focal 
length of the lens is nearly equal to C S -j- 
S T, i. e. to the sum of the distances of the 
lens from the middle of the mirror, and of 
the middle of the mirror from the bottom 
of the larger box. The lens being turned 
toward the prospect would form a picture 
of it, nearly at its focus ; but the rays be- 
ing intercepted by the mirror will form the 
picture as far before the surface as the focus 
is behind it, i. e. at the bottom of the 
larger box, a communication being made be- 
tween the boxes by the vacant space Q O. 
The draughtsman then putting his head and 
hands into the box through the open side, 
D C, and drawing a curtain round to pre- 
vent the admission of the light, which 
would disturb the operation, may trace a 
distinct outline of the picture that appears 
on the bottom of the box. 
There is another kind of camera obscura, 
constructed thus. In the extremity of the 
