Physiology and Hygiene for Secondary Schools

by Francis M. Walters

[pg 371]Kinds of Reflection.—Waves of light striking against the smooth surface of a mirror are thrown off in definite directions, depending on the angle at which they strike. (Illustrate by holding a mirror in the direct rays of the sun.) But light waves that strike rough surfaces are reflected in practically all directions and apparently without reference to the angle at which they strike. (Illustrate by placing a piece of white paper in the direct rays of the sun. It matters not from what direction it is viewed, waves of light strike the eye.) This kind of reflection is called diffusion, and it serves the important purpose of making objects visible. The light waves passing out in all directions from objects which have received light from the sun, or some other luminous body, enable them to be seen.

Fig. 157—Diagram illustrating passage of light waves.On the right the light is transmitted by the glass, reflected by the mirror, refracted by the prism, and absorbed by the black cloth. On the left the light from the candle forms an image by passing through a small hole in a cardboard and falling upon a screen.

Formation of Images.—Another principle necessary to seeing is that of refraction. Refraction means the bending, or turning, of light from a straight course. One of the most interesting effects of refraction is the formation of images of objects, such as may be accomplished by light from them passing in a certain manner through convex lenses. If, for example, a convex lens be moved back and forth[pg 372] between a candle and a screen in a dimly lighted room, a position will be found where a picture of the candle falls upon the screen. This picture, called the image, results from the refraction of the candle light in passing through the lens.

Fig. 158—Diagram illustrating formation of images. On the right the image is formed by a double convex lens; on the left by the lenses of the eye. The candle flame represents a luminous, or light-giving, body; but light passes from the large arrow by reflection. (See text.)

In order to form an image, the light waves spreading out from the object must be brought together, or focused. Focusing means literally the bringing of light to a point, but it is evident in the formation of an image that all the waves are not brought to a single point. If they were, there would be no image. In the example of the candle given above, the explanation is as follows:

The light from the candle comes from a great number of separate and distinct points in the candle flame. The lens, by its peculiar shape, bends the waves coming from any single point so that they are brought to a corresponding point on the screen. Furthermore, the points of focused light are made to occupy the same relative positions on the screen as the points from which they emanate in the candle flame (Fig. 158). This is why the area of light on the screen has the same form as the candle, or makes an image of it. The same explanation applies if, instead of the luminous candle, a body that simply reflects light, as a book, is used.

The Problem of Seeing.—What we call seeing is vastly more than the stimulation of the brain through the action of light upon afferent neurons. It is the perceiving of all the different things that make up our surroundings. If[pg 373] one looks toward the clear sky, he receives a sensation of light, but sees no object. He may also get a sensation of light with the eyelids closed, if he turn the eyes toward the window or some bright light. But how different when the light from various objects enters the eyes. There is apparently no consciousness of light, but instead a consciousness of the size, form, color, and position of the objects. Seeing is perceiving objects. Stimulation by the light waves is only the means toward this end. The chief problem in the study of sight is that of determining how light waves enable us to become conscious of objects.

Sense Organs of Sight.—The sense organs of sight consist mainly of the two eyeballs. Each of these is located in a cavity of the skull bones, called the orbit, where it is held in position by suitable tissues and turned in different directions by a special set of muscles. A cup-shaped receptacle is provided within the orbit, by layers of fat, and a smooth surface is supplied by a double membrane that lies between the fat and the eyeball. In front the eyeballs are provided with movable coverings, called the eyelids. These are composed of dense layers of connective tissue, covered on the outside by the skin and lined within by a sensitive membrane, called the conjunctiva. At the base of the lids the conjunctiva passes to the eyeball and forms a firmly attached covering over its front surface. This membrane prevents the passage of foreign materials back of the eyeball, and by its sensitiveness stimulates effort for the removal of irritating substances from beneath the lids. The eyelashes and the eyebrows are also a means of protecting the eyeballs.