PRINCIPLES OF VISION.
Through the medium of our eyes we see objects by means of the light
which is reflected from them. This light enters the eye and forms an
inverted image of the object upon the retina, just as an inverted image
is formed upon the ground glass of a camera. This impression made upon
the eye is corrected automatically, so that, although we see everything
upside down, we are not at all aware of so doing. The proof of this
peculiarity of the eye is found in cases where persons born blind have
later through operations acquired sight. In the eyes of slaughtered
cattle also the image can be seen inverted. A further proof that we are
able thus to adjust ourselves is found in the experience of persons
using cameras with ground glass screens. The image on these screens is
always inverted both horizontally and vertically. The user soon learns
to see his object, although inverted, in the natural way, that is,
vertically, because this is so plain that he must take it into account
at every focusing. He does not, however, accommodate himself to the
reversal from right to left because this is of no consequence ordinarily
and is not noticed. Many photographers, who have been accustomed to the
vertical inversion, still find themselves confused when trying to locate
the right and left of a view seen through the lens.
Unless a special arrangement of lenses is provided, all images cast
upon screens through small openings appear inverted. The reason for this
can be seen from Figure 62. It is obvious that no other light can reach
the bottom of the screen through the pin hole _O_, at the left of the
figure, except that coming from the flame of the candle; also that no
other light can reach the top of the screen except that reflected from
the bottom of the candle at the right. Hence the image of the candle
appears inverted.
[Illustration: FIGURE 62.]
[Illustration: FIGURE 63.]
A general understanding of the structure of the eye can be had from
Figure 63. _W_ indicates a watery substance in the front of the eye; _I_
is the iris which has power to contract or expand and thus regulate the
quantity of light admitted to the eye; _P_ is the pupil; _L_ is the
lens; and _R_ is the retina which connects with the optic nerve and the
brain. The lens is made up of several parts having different indexes of
refraction. The whole resembles an ordinary convex lens but has
considerable power of adjustment. When looking at objects close by, for
instance, the pupil can often be seen to bulge out which is its method
of accommodating itself to objects close at hand.
To the iris falls the duty of regulating the quantity of light which is
to reach the retina. If confronted by a bright light, it closes
partially; in a dim light, it opens out wide. When subject to a
flickering light, there is a tendency to follow the flickerings by rapid
opening and closing, which causes pain. If subject to flickerings long
enough, however, the pain becomes somewhat less, probably because the
iris has come to rest on an intermediate point.
An image formed upon the retina remains for some time, the time varying
with the intensity of the light. Very intense impressions are supposed
to last about one twenty-fifth of a second; milder ones as long as
one-tenth of a second. This tendency to retain images is known as the
_persistence of vision_ and can be noticed in many ways. A twenty-five
cycle alternating current falls to zero fifty times in one second; and
fifty times in each second there is a slight cooling off of the
incandescent filament. Yet the variation in the intensity of the light
is noticeable. Many of the sleight-of-hand tricks depend upon this
persistence of vision and the projection of moving pictures would be
impossible without it.
We are able to judge distance principally through the fact that we have
two eyes. If our eyes were immovable, we should see two images for
every object. But as they are movable and as both normally point
directly at the object we are looking at, their axes form angles with
each other and in this way we are enabled to judge the distance, as well
as other qualities of objects.
When both eyes are centered upon an object, the impressions received by
the brain from both sources are mixed, and the picture we become
conscious of is a composite of the two images in the eyes.
This is verified by the fact that many persons with defective vision can
see much more clearly with one eye than with both. They are not able to
focus both eyes upon the same point and thus the perfectly clear picture
which may exist in one eye is mixed with an uncertain picture in the
other.
In youth normal eyes are able to adjust themselves to different
intensities of light and different distances very rapidly. This power is
largely curtailed as age advances. Where a young person can almost
instantly, after gazing at some distant object, turn to a newspaper and
read, the eyes of a person of advanced age generally require
considerable time before they can adjust themselves in the same way.
Quite frequently, however, very old people regain their powers of vision
and become able to do without the glasses formerly used.
All of the above facts should be thoroughly understood by those having
to do with illumination used by a mixed audience. Light that may seem
perfectly satisfactory to one may be entirely unsuitable for another.