No, in all probability, says the reader; but the opposite popular
belief is supported by eminent naturalists.
Buffon says: “The eyes of the cat shine in the dark somewhat like
diamonds, which throw out during the night the light with which
they were in a manner impregnated during the day.”
Valmont de Bamare says: “The pupil of the cat is during the night
still deeply imbued with the light of the day;” and again, “the
eyes of the cat are during the night so imbued with light that they
then appear very shining and luminous.”
Spallanzani says: “The eyes of cats, polecats, and several other
animals, shine in the dark like two small tapers;” and he adds that
this light is phosphoric.
Treviranus says: “The eyes of the cat _shine where no rays of
light penetrate_; and the light must in many, if not in all, cases
proceed from the eye itself.”
Now, that the eyes of the cat do shine in the dark is to a certain
extent true: but we have to inquire whether by _dark_ is meant the
entire absence of light; and it will be found that the solution of this
question will dispose of several assertions and theories which have for
centuries perplexed the subject.
Dr. Karl Ludwig Esser has published in Karsten’s Archives the results
of an experimental inquiry on the luminous appearance of the eyes of
the cat and other animals, carefully distinguishing such as evolve
light from those which only reflect it. Having brought a cat into a
half-darkened room, he observed from a certain direction that the cat’s
eyes, when _opposite the window_, sparkled brilliantly; but in other
positions the light suddenly vanished. On causing the cat to be held
so as to exhibit the light, and then gradually darkening the room, the
light disappeared by the time the room was made quite dark.
In another experiment, a cat was placed opposite the window in a
darkened room. A few rays were permitted to enter, and by adjusting the
light, one or both of the cat’s eyes were made to shine. In proportion
as the pupil was dilated, the eyes were brilliant. By suddenly
admitting a strong glare of light into the room, the pupil contracted;
and then suddenly darkening the room, the eye exhibited a small round
luminous point, which enlarged as the pupil dilated.
The eyes of the cat sparkle most when the animal is in a lurking
position, or in a state of irritation. Indeed, the eyes of all animals,
as well as of man, appear brighter when in rage than in a quiescent
state, which Collins has commemorated in his Ode on the Passions:
“Next Anger rushed, his eyes on fire.”
This brilliancy is said to arise from an increased secretion of the
lachrymal fluid on the surface of the eye, by which the reflection of
the light is increased. Dr. Esser, in places absolutely dark, never
discovered the slightest trace of light in the eye of the cat; and he
has no doubt that in all cases where cats’ eyes have been seen to shine
in dark places, such as a cellar, light penetrated through some window
or aperture, and fell upon the eyes of the animal as it turned towards
the opening, while the observer was favourably situated to obtain a
view of the reflection.
To prove more clearly that this light does not depend upon the will of
the animal, nor upon its angry passions, experiments were made upon
the head of a dead cat. The sun’s rays were admitted through a small
aperture; and falling immediately upon the eyes, caused them to glow
with a beautiful green light more vivid even than in the case of a
living animal, on account of the increased dilatation of the pupil.
It was also remarked that black and fox-coloured cats gave a brighter
light than gray and white cats.
To ascertain the cause of this luminous appearance Dr. Esser dissected
the eyes of cats, and exposed them to a small regulated amount of light
after having removed different portions. The light was not diminished
by the removal of the cornea, but only changed in colour. The light
still continued after the iris was displaced; but on taking away the
crystalline lens it greatly diminished both in intensity and colour.
Dr. Esser then conjectured that the tapetum in the hinder part of the
eye must form a spot which caused the reflection of the incident
rays of light, and thus produce the shining; and this appeared more
probable as the light of the eye now seemed to emanate from a single
spot. Having taken away the vitreous humour, Dr. Esser observed that
the entire want of the pigment in the hinder part of the choroid coat,
where the optic nerve enters, formed a greenish, silver-coloured,
changeable oblong spot, which was not symmetrical, but surrounded the
optic nerve so that the greater part was above and only the smaller
part below it; wherefore the greater part lay beyond the axis of
vision. It is this spot, therefore, that produces the reflection of the
incident rays of light, and beyond all doubt, according to its tint,
contributes to the different colouring of the light.
It may be as well to explain that the interior of the eye is coated
with a black pigment, which has the same effect as the black colour
given to the inner surface of optical instruments: it absorbs any
rays of light that may be reflected within the eye, and prevents
them from being thrown again upon the retina so as to interfere with
the distinctness of the images formed upon it. The retina is very
transparent; and if the surface behind it, instead of being of a dark
colour, were capable of reflecting light, the luminous rays which had
already acted on the retina would be reflected back again through it,
and not only dazzle from excess of light, but also confuse and render
indistinct the images formed on the retina. Now in the case of the cat
this black pigment, or a portion of it, is wanting; and those parts of
the eye from which it is absent, having either a white or a metallic
lustre, are called the tapetum. The smallest portion of light entering
from it is reflected as by a concave mirror; and hence it is that the
eyes of animals provided with this structure are luminous in a very
faint light.
These experiments and observations show that the shining of the eyes
of the cat does not arise from a phosphoric light, but only from a
reflected light; that consequently it is not an effect of the will of
the animal, or of violent passions; that their shining does not appear
in absolute darkness; and that it cannot enable the animal to move
securely in the dark.
It has been proved by experiment that there exists a set of rays of
light of far higher refrangibility than those seen in the ordinary
Newtonian spectrum. Mr. Hunt considers it probable that these highly
refrangible rays, although under ordinary circumstances invisible
to the human eye, may be adapted to produce the necessary degree
of excitement upon which vision depends in the optic nerves of the
night-roaming animals. The bat, the owl, and the cat may see in the
gloom of the night by the aid of rays which are invisible to, or
inactive on, the eyes of man or those animals which require the light
of day for perfect vision.
Astronomy.