Physiology: The Science of the Body

by Ernest G. Martin

accommodation the focus can be thrown forward, most persons with short eyeballs can see distant objects clearly by accommodating for them, and near objects that are not too near by extreme accommodation. For this reason hyperopia, as this condition is called, is usually not discovered until the person begins to feel the strain of the constant accommodation that is necessary whenever the eyes are open. Eyestrain usually shows itself in headaches; in fact, so large a proportion of headaches come from this cause that anyone who suffers from them at all frequently should have his eyes examined by a competent oculist. Relief for hyperopia is by means of glasses that shorten the focus and thus bring the image of distant objects forward to where the retina is in the short eyeball.

There is one other defect of vision that is so common as to call for a word; this is astigmatism. It is the condition in which the cornea is not curved equally in all directions; the vertical curvature may be greater or less than the horizontal. The effect is that points on objects do not focus sharply as points in the image, but as little elliptical spots. If one adjusts the accommodation so that the top and bottom edges of objects are sharp the sides will be blurred and vice versa. Usually the blurring is not great enough to be noticeable, but only enough to make the person unconsciously dissatisfied with the accommodation. He, therefore, constantly tries to improve it by changing the tension of his ciliary muscles, and so brings on eyestrain. The correction for this condition is glasses that are not equally curved in all directions, but so selected that their less curvatures shall fit in with the greater curvatures of the cornea. In fact, glasses that are curved only in one direction are usually used, this curvature being just enough to bring up the total curvature in that direction to equal the other curvatures of the cornea. It ought not be necessary to give warning that only competent persons should be allowed to examine and prescribe for the eyes. Great skill is needed to determine accurately just how far from correct the eyeball is, and unless this is known there is no means except guesswork by which to decide on a prescription.

The third feature of vision is the perception of color. Color is to light what pitch is to sound; that is, it depends on the vibration rate of the light waves. Light, as already explained, is one of the forms in which energy reaches the earth from the sun. Heat is another form. Both are portions of a great energy stream to which we give the name of radiant energy. This, as it comes from the sun, is made up of a mixture of vibrations having almost every imaginable rate, except that the slowest are many times faster than the highest pitched sound. At a certain rate, and one that for these vibrations ranks as slow, the energy is what we know as heat, and over a considerable range it continues to be called heat; more rapid vibrations, and they are so rapid that they have to be expressed in trillions per second, cause the effect on our retinas that we call light. The slowest that we can see give the sensation of red; the most rapid the sensation of violet; the other colors of the rainbow, which in order after red are orange, yellow, green and blue, are vibration rates between those that give red and those that give violet. It will be noticed that only six colors are given for the rainbow instead of seven. This is because there is not enough real difference between blue and indigo to justify making them separate colors. The distinction was made at the time when it was supposed that there was something specially wonderful about the number seven, which made it necessary that every important feature in nature should show that number. We now know of no reason why seven should have virtue over any other number. When all the vibration rates are mixed together, as they are in the sunlight, the sensation is white. There are other mixtures of colors that give white also, but they are not exactly equivalent to the white of sunlight, as is proven when one tries to match colors under artificial light. A great deal of labor has been devoted to the attempt to get an artificial light that shall be practically equivalent to sunlight, and only lately have good results been obtained. When no light enters the eye the sensation is of black, and it is worth while to note that so far as our sensations are concerned black is as much a color as white or any other, notwithstanding the fact that no light falls on the retina when the black sensation is being felt. Contrary to ordinary belief, blind persons who are blind because their eyes have been destroyed do not see black all the time; they simply have no sensations at all from the eyes. On the other hand, persons blind because of cataract do see black, because in them the retinas are still present, but no light falls on them.

The perception of color is very complicated, and not at all well understood. Persons who do not have the same color perception as most of us are called color-blind, and by learning some things about color-blindness we shall best get an idea of color perception itself. About four men in one hundred have defective color sense; the proportion in women is only about a tenth as great. By far the commonest type is one in which neither red nor green is seen correctly, but both are seen as neutral tints, and in many cases look so much alike that the person cannot tell one from the other. The practical importance of knowing whether or not this defect is present is seen when we think that red and green lights are used more than any other colors in signaling, so that railroad men and others who work by signals must have normal color sense. It has been discovered that even people that have normal color vision are color-blind in the margins of the retina. This can easily be demonstrated by bringing a red or green disk slowly around in front of the eye of a person who, meanwhile, keeps looking straight ahead. He will see the disk out of the corner of his eye some time before he can tell what color it is. In fact, if a red or green disk is used, he usually will not be certain as to the color until it is almost straight in front. Blue or yellow disks can be told with certainty much farther out, but even these colors are not perceived clear to the edge of the field of vision. These experiments show that the retina becomes more and more highly developed as an organ for perceiving color as we get closer and closer to the center; at the extreme edges there is no color sense at all, but only the primitive ability to tell light from darkness; closer in blue and yellow are distinguished and not red and green; only in the central part are all colors clearly seen.