OPTICS, 
for the course of the reflected ray, D B. All 
these reflected rays will meet in point d, 
where they will form the extremity, d, of 
the inverted image, e d, similar to the ex- 
tremity, D, of the upright object, D E. If 
the pencil of rays, E f, Eg, E h, be also 
continued to the mirror, and their angles of 
reflection from it be made equal to their 
angles of incidence upon it, as in the for- 
mer pencil from D, they will meet at the 
point, €, by reflection, and form the extre- 
mity, e, of the image, e d, similar to the 
extremity, E, of tire object, D E. As each 
intermediate point of the object between 
D and E, sends out a pencil of rays in like 
manner to every part of the mirror, the 
rays of each pencil will be reflected back 
from it, and meet in alt the intermediate 
points between the extremities, e and d, of 
the image; and so the Whole image will be 
formed not at i, half the distance of the 
mirror from its centre of concavity, C ; 
but at a greater distance between i and 
the object, D E ; and the image will be in- 
verted with respect to the object. This 
being well understood, the reader will ea- 
sily see Iww the image is formed by the 
large concave mirror of the reflecting tele- 
scope, when he comes to the description of 
that instrument See Telescope. 
When the object is more remote from 
the mirror than its centre of concavity, C, 
the image will be less than the object, and 
between the object and the mirror; when 
the object is nearer than the centre of con- 
cavity, the image will be more remote, and 
bigger than the object: thus, if D E be the 
object, e d will be its image ; for as the ob- 
ject recedes from the mirror, the image 
approaches nearer to it; and as the object 
approaches nearer to the mirror, the image 
recedes further from it ; on account of the 
lesser or greater divergency of the pencils 
of rays which proceed from the object ; for 
the less they diverge, the sooner they are 
converged to points by reflection ; and the 
more they diverge, the further they must 
be reflected before they meet. If the ra- 
dius of the mirror’s concavity and the dis- 
tance of the object of it be known, the 
distance of the image from the mirror is 
found by this rule ; Divide the product of 
the distance and radius by double the dis- 
tance made less by the radius, and the quo- 
tient is the distance required. If the ob- 
ject be in the centre of the mirror’s conca- 
vity, the image and object will be coinci- 
dent, and equal in bulk. 
If a man place himself directly before a 
large concave mirror, but further from It 
than its centre of concavity, he will see an 
inverted image of himself in the air, be- 
tween him and the mirror, of a less size 
than himself. And if he hold out his hand 
towards the mirror, the hand of the image 
will come out towards his hand, and coin- 
cide with it, of an equal bulk, when his 
hand is in the centre of concavity ; and he 
will imagine he may shake hands with his 
image. If he reach his hand further, the 
hand of the image will pass by his hand, 
and come between it and his body ; and if 
he move his hand towards either side, the 
hand of tlie image will move towards the 
other; so that whatever way the object 
moves, the image will move the contrary 
way. A by-stander will see nothing of 
the image, because none of the reflected 
rays that form it enter his eyes. 
The images formed by convex specula 
are in positions similar to those of their 
objects ; and those also formed by concave 
specula, when the object is between the 
surface and the principal focus : in these 
cases the image is only imaginary, as the 
reflected rays never come to the foci from 
whence they seem to diverge. In all other 
cases of reflection from concave specula, 
the images are in positions contrary to those 
of their objects, and these images are real, 
for the ray after reflection do come to their 
respective focL These things are evident 
from what has gone before. See Mirror. 
“ Of colours and the different refrangibi- 
lity of light.” Tire origin of colours is ow- 
ing to the composition which takes place in 
the rays of light, each heterogeneous ray 
consisting of innumerable rays of different 
colours ; this is evident from the separation 
that ensues in the well-known experiment 
of the prism. A ray being let into a dark- 
ened room (fig. 11) through a small round 
aperture, z, and falling on a triangular 
glass prism, x, is by the refraction of the 
prism considerably dilated, and will ex- 
hibit on the opposite wall an oblong image, 
a b, called a spectrum, variously coloured, 
the extremities of which are bounded by 
semicircles, and the sides are rectilinear. 
The colours are commonly divided into se- 
ven, which, however, have various shades, 
gradually intermixing at their juncture. 
Their order, beginning from the side of the 
refracting angle of the prism, is red, orange, 
yellow, green, blue, purple, violet. The ob- 
vious conclusion from this experiment is, 
that the several component parts of solar 
light have different degrees of refrangibility, 
