OPTICAL IMAGES. 
reflection already explained. If b d be at right angles to m n, 
the angle, D B o, will he equal to B a n, and also to D B A, 
and consequently to b a n, from whence it follows that B A 
F . 4 is eqnal to B a , and a n to 
(in: and since the same 
will he true of all rays 
which issue from a towards 
the reflector, it follows 
that, after reflection, all 
such rays will enter the 
eye, o, as if they had di¬ 
verged from a . 
The eye o will therefore 
see the point a in the re*r 
Hector as if it were at a. 
7. But since the same 
will he true of each point 
in an object, A b (fig. 5), 
placed before the reflector, 
it follows that the rays 
which proceed from the 
several points of the object will, after reflection, enter the eye, as 
if they came from corresponding points of a similar object a b , 
placed just as far behind the reflector 
as the object itself A b is before it. 
It is evident that in this case the 
image a b is not only similar to the 
object but precisely equal to it. Its 
position relatively to the reflector is 
similar to that of the object, but in 
an absolute sense it is different, as 
will be evident from observing that 
while the arrow A b points to the left, 
its image a b points to the right. 
8. It will be perceived, that the 
reflected rays by which the perception 
of the image is produced, do not 
actually form the image. They enter 
the eye as if they actually came from 
the several points of such an image 
as the eye sees, but they do not come from such points. In 
such cases, where the image is perceived, but not actually pro¬ 
duced, it is called a virtual or imaginary image . When the rays 
by which the image is perceived do actually diverge from the 
points of the image, the image is said to be eeal. 
Figr .*5. 
86 
