IMAGES BY MIRRORS. 
divisions being numbered from that point in each direction 
towards c and c\ Let a small reflector (a piece of looking-glass 
will answer the purpose) be placed upon the horizontal diameter 
at the centre with its re¬ 
flecting surface down¬ 
wards, and let any con¬ 
venient and well-de- 
flned object be placed 
upon the graduated 
arch at any point, such 
as a , between d and c. 
Now, if the point a' be 
taken upon the arch d c 
at a distance d a' from d 
equal to d a, the eye 
placed at a' and directed 
to b will perceive the object a as if it were placed in the direction 
a' b. It follows, therefore, that the light issuing from the point 
of the object a in the direction a b, is reflected to the eye in 
the direction b a\ In this case, the angle a b d is the angle of 
incidence, and the angle d b a* is the angle of reflection; and, 
whatever position may be given to the object a, it will be found 
that, in order to see it in the reflector £, the eye must be placed 
upon the arch d c\ at a distance from d equal to the distance at 
which the object is placed from d upon the arch d c. 
The same principle may also be experimentally illustrated as 
follows:— 
If a ray of sun-light admitted into a dark room through a small 
hole in a window-shutter strike upon the surface of a mirror, it 
will be reflected from it, and both the incident and reflected rays 
will be rendered visible by the particles of dust floating in the 
room. By comparing the direction of these two visible rays with 
the direction of the plane of the mirror and the position of the 
point of incidence, it will be found that the law of reflection which 
has been announced is verified. 
6. This being premised, it will be easy to comprehend the 
manner in which images are produced by reflection from plane 
surfaces. 
Let a, fig. 4, be any point of a visible object placed before a 
plane reflector, m n. Let A b and a c be two rays diverging from 
it, and reflected from b and c to an eye at o. After reflection, 
they will proceed as if they had issued from a point, a, as far 
behind the reflector as the point, A, is before it; that is to say, 
the distance a n will be equal to a N. 
It is easy to verify this, by taking into account the law of 
85 
