This relation between the blood-vessels of the iris and its movements has many important advantages; as, for instance, during sleep, when the vessels dilate, the pupil contracts, thus preventing the light from being felt too vividly. In inflammation of the eye, light exercises an irritating and injurious influence; but the vessels during inflammation are always dilated, the pupil is therefore narrower, and the light which strikes the back of the eye less intense, recovery being consequently more speedy.
After copious loss of blood, in fatigue, deep depression, in pain, and similar cases, the vessels contract and the pupil dilates, in this way allowing many things to be seen which would be imperceptible for want of light if the pupil were contracted.
All this seems perfect as an apparatus, but unfortunately it has grave defects.
Our eye is like a photographic machine, and the pupil acts like the diaphragm which photographers put before the lens, for in our eye, too, there is a lens similar to that of the photographic camera, behind the diaphragm of the iris. When there is little light, the photographer puts in a diaphragm with a wider opening, but then the picture becomes dull, because the rays of light in passing further from the centre of the lens and on its peripheral edge produce a picture with indistinct outlines. Photographers, therefore, in order to obtain a picture clear in all its parts, prefer a very strong light, and make use of a diaphragm with a very small opening. These are also the best conditions for distinct vision; for if we observe the eyes of a person who is looking into distance, or is absent-minded, and then hold a small object before him, we see that the pupil immediately contracts.
But this wonderfully perfect mechanism ceases to act as soon as the animal or the man is subjected to violent emotion. When the vessels contract during fear or a struggle, or in any other exertion, the pupil immediately dilates, and the picture loses in distinctness. If we watch fighting dogs, cats, or men, we at once perceive that the eye has become blacker, and that the pupil is at its maximum dilatation.
But how shall we explain, by means of the hypothesis of Spencer and Darwin, the fact that nocturnal animals present with equal precision the same movements in the expression of the forehead and eyes? Why, for the sake of such a small advantage as being able to see a little better when we have the light in our eyes, is there such a complicated muscular apparatus always in operation, while nature has not provided against a far more serious defect, as is the confusion of images caused by the too great dilatation of the iris?
In order to appreciate the extent of the defect of vision during emotion, I made the following experiment, together with Dr. Falchi. We took a small sample of writing from Snell’s tables, and then determined what was the greatest distance at which it could be easily read by a certain person; then, on some pretext, we scolded or reproached the subject in such a manner as to occasion a sudden and strong emotion. When we then requested the same person to read the writing, he was no longer able to do so at the same distance, but had to approach the tablet, often by a few steps, in order to see as before. A violent muscular exertion, a few turns on the trapeze or in the gymnastic rings, a race, the rapid ascent of a staircase, also diminish the acuteness of vision in a noticeable manner.
III
When one considers as a whole the symptoms by which fear reveals itself, one might almost think that it was a product of heredity and selection. Animals that are easily frightened, a disciple of Darwin would say, are those which can more easily avoid danger and save themselves; these produce young, and perpetuate their timidity in their posterity. But we know that the phenomena of fear are the morbid exaggeration of physiological facts. Animals cannot become continually more timorous by means of hereditary transmission; the necessity of struggling brings other faculties than those of flight and fear into play, and effect the preservation of the species in another way. Our organism is not such a perfect machine as to be able to resist or adapt itself to all conditions of environment; there are inevitable necessities against which selection is of no avail.