J0CULA® IMAGE. 
This will be rendered more clear by reference to fig. 1. Let 
A, B, c, be a candle, for example, placed before the eye, e. Lays 
diverge from the top, A, of the flame, and enter the pupil. A cone 
of these rays, whose point is at A, and whose, base is the pupil, 
enter the eye, and being collected on the retina, produce a percep¬ 
tion of the point A.* And other cones, or pencils, as they are 
called, proceeding from the points b and c, and, in general, from 
all the points of the candle, radiate to the pupil in like manner, 
and severally produce perceptions, and so a perception of the candle 
is produced. 
Now, if a, b, c, instead of being a real candle, were merely the 
optical image of a candle, the same perception of its presence 
would be produced, provided the same rays radiated in like 
manner from each point to the eye, and the observer would see 
it exactly as he would see the object itself, were it in the same 
position. 
But it is not even necessary to the production of the. perception 
that either the objector its image should be present, if the rays, 
no matter where they may have originated, or what route they 
may have followed, only enter the eye in the same lines of direc¬ 
tion which they would have, had they come directly from the 
object. Thus, for example, if the pencils, instead of coming from 
A and c, had come from a similar point at a' and c' towards a and 
e, and had there by any optical agency been turned into the direc¬ 
tions which they would have had, if they had come from A and o 
to the pupil, the perception produced by them would be exactly 
the same. 
In fine, the perceptions produced depend on the directions 
which the rays have in entering the pupil, and are altogether 
independent of the route they may have followed before arriving 
there. 
It will be most necessary that this fact be impressed on the 
memory, since the whole theory of vision, especially where optical 
agents are used, depends more or less upon it. 
3. IMAGES PRODUCED BY PLANE REFLECTORS. 
The most simple case of the production of optical images, and 
that of most frequent occurrence, is when they are produced by 
reflection from plane surfaces; as when a landscape and the firma¬ 
ment are seen reflected in the surface of water, or when objects arc 
seen in a looking-glass. 
To explain this very familiar phenomenon, it is necessary first 
* See Tract on The Eye, vol. v., pp. 54, 55. 
o 2 
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