Light travels through any even transparent body, such as the atmosphere, in a straight line. But when it meets any change in density, particularly when it has to pass obliquely into a denser medium, the ray is bent so as to run in a direction more perpendicular to the surface of the denser body. The degree of bending or refraction of the rays depends on the difference in optical density of the two media and the angle at which the ray strikes the surface of the more dense.
Fig. 222. Diagram showing the course of parallel rays of light from A in their passage through a biconvex lens L, in which they are so refracted as to bend toward and come to a focus at a point F.
On its way to the sensitive retina, the light has to pass through the various transparent media just named, viz., the cornea, the aqueous humor, the crystalline lens, and the vitreous humor. On entering these media, which have different densities, the rays of light emitted by any luminous body become bent or refracted, so that they are brought to a focus on the retina, just in the same way as parallel rays of light from the sun may be focused on a near object by means of an ordinary convex lens.
Fig. 223. Diagram showing the course of diverging rays, which are bent to a point further from the lens than the parallel rays in last figure.
Only so much light reaches the fundus of the eye as can pass through the opening in the iris, so that a comparatively narrow and varying beam is admitted to the chamber in which the nerve endings are spread out for its reception.
If we hold a biconvex lens at a certain distance from the eye and look out of the window through it, we see an inverted image of the landscape. If we place a piece of transparent paper behind the lens, we can throw a representation of the picture on it, which will be seen to be inverted. This power of convex lenses is employed in the instrument used for taking photographs, called a camera, which consists of a box or chamber into which the light is allowed to pass through a convex lens, so that an inverted image of the objects before it is thrown upon a screen of ground glass within the box. When the sensitive plate replaces the screen, the light coming through the lens makes a photograph.
Just in the same way an inverted image of the things we look at is thrown on the retina by the refracting media of the eye. This may be seen in a dark room, if a candle be placed at a suitable distance in front of the cornea of an eye taken from a recently killed white rabbit. When cleared of fat and other opaque tissues, the sclerotic is transparent enough to act as a screen upon which the inverted candle flame can be recognized.
Though our organ of vision is often compared to a camera obscura, the refractions of light which occur in it are far more complex than those taking place in that simple instrument. In the latter we have only two media - the glass lens and the air; in the eye, on the other hand, we have several, which are known to have a distinct refractive influence on the rays which pass through the pupil.