LIGHT AND ITS PERCEPTION
The principal means by which most animals are made aware of their
surroundings, and changes in these surroundings, is the reflection or
emission of light toward them by external objects and the reception of
this light by special organs which we term photoreceptors. The more
complicated of these photoreceptors are called eyes, though it is not
complexity, as such, which governs the applicability of that special term.
We say that the function of the eye is vision, but since all photoreception
is not vision and not all photoreceptors are eyes, we must consider these
broader and narrower terms before delving into our subject proper —
the structure and variations of vertebrate eyes and their relation to the
ways of life of their possessors.
Light may best be defined, for our purposes here, as a rhythmic eman-
ation of energy whose rhythm-frequency or pitch falls within definite
limits, outside of which are the higher or lower frequencies of radio,
cosmic, X-, and other rays. Visible light thus forms a circumscribed
band of frequencies to which the eye happens to be sensitive and which,
compared with all forms of radiant energy in general, is like a single
octave toward the high-pitched end of the scale of a piano (see Table I) .
It contains only a small fraction of the total amount of energy given off
by the sun, and sunlight in turn forms only a portion of the 'grand
spectrum' of radiant energy. Like other forms of radiant energy, light
in its ultimate units can vary in but simple ways — in speed, in frequency,
and in intensity. But natural lights and illuminations are complex mix-
tures of these variations, and make possible the infinite variety of nature's
pictures, varying in tone or shading (owing to combinations of inten-
sities) and in color or hue (owing to combinations of frequencies) ,
We have been discussing light as an objective physical entity; but, just
as there would be no sound if a tree were to fall with no one to hear it,
so also there would be no light in the physiological sense if there were no
photoreceptor upon which it impinged. In this other sense light is a
sensation, an experience in consciousness. Like other such experiences,
it may be evoked by a limited number of causes (other than actual physical light) . The qualities of a light-sensation bear only a close, not
an absolute, relationship to the objective attributes of a physical light
which produces it. Thus, different colors may be seen under special cir-
cumstances when the corresponding different frequencies of light are
not being steadily presented to the eye at all, or the same color may
result from totally different mixtures of frequencies. Two lights with the
same energy-content may appear different in brightness while two others,
equally bright, may differ greatly in actual physical intensity. Color and
brightness are thus subjective correlates of the objective frequency and
intensity. The former can be perceived but not measured, while the latter
can be measured with inanimate instruments but cannot be perceived
with the eye.
A sobering array of optical illusions may be seen by the reader in any
good reference work on psychology, and will serve to teach, still more
emphatically, the lesson that: "Our eyes do not see; but we see with our
eyes." Photoreception is one thing — it may be conscious, the reception
of the external stimulus of light upon the sill of the "window of the
soul" — or it may lead reflexly to quite unconscious activities such as the
change of the size of the pupil, the aiming of the eyes, the blinking of
the lids when the eye is about to be struck by something, and so on.
Vision is something more. It is the complex and sometimes deceptive
product of the interaction of the simple information which travels along
the optic nerve and the manipulations, as yet unfathomable, which this
information undergoes in the brain before it is presented to the con-
sciousness for action or other disposal.
A photoreceptor may be constituted by a single part of a one-celled
animal; by one of a number of similar, scattered, photosensory cells in
an invertebrate's skin; by a patch of cells closely aggregated into a plate,
or lining a pit; or by an ocellus or eye (Fig. 1). This last term is best
reserved for those photoreceptors in which there is a light-sensitive layer
of cells upon which accessory parts converge the light rays received from
environmental objects. An eye, then, ordinarily contains at least a photo-
sensory epithelium or retina, and a lens. An image may however be
formed upon the retina by a pinhole (as in the chambered nautilus)
instead of by a lens; or, the lens in a given type of eye may be employed
to concentrate the light in order that the eye may work in dimmer illum-
inations, instead of to form an image so that the mind may have a picture.
Finally, a number of 'concentrator' units may be congregated so that a
mosaic image can be built up in the consciousness itself, and it is upon
this plan that the 'compound' eyes of many arthropods are constructed.
Vertebrate eyes are all built upon one fundamental plan. With the
exception of those which have degenerated because their owners live
underground, or in the perpetual night of caves or
the depths of the
ocean, they are provided with a retina and with a ~
lens whose optical
properties are such that it forms an image upon the retina. The lenses of
the median eyes which some reptiles possess on
the top of the head are
probably often of the concentrator type; but those of the lateral, or
ordinary, eyes are nearly always eikonogenic —
that is, image-forming.
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