*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 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] 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 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._