To begin with, then, the retinal nerves are strictly reserved to respond to the vibrations of ether—called light. If the student has ever had a blow on his eye, he has probably seen “stars,” because every stimulus to this pair of nerves makes us see things, and not feel them. Now each sense has certain limits between which it can detect subtle vibrations, but beyond which all is blank. The more refined the organization of the person, the greater will be the number of vibrations he can distinguish. Thus 399,000,000,000 vibrations in a second produce in us the sensation of light, above this the vibrations appear as spectral colours until the number 831,000,000,000,000 is reached; to an increase in the number of vibrations above that number the optic nerve does not respond. Now the eye is an optical apparatus fixed between the brain and the ether, not that we may perceive light, for we could do that without the eye, but that we may distinguish objects. The glyptic and pictorial arts are founded entirely on the sense of sight as music is founded on the sense of hearing. In the pictorial arts, then, we must clearly distinguish between the physical, physiological, and psychological properties of sight.
Le Conte’s division.
Le Conte divides the scientific, i.e. physical and physiological data, into: A. Light; B. Direction of Light; C. Intensity; D. Colour; and the psychological data into Binocular vision, size, solidity, and depth. Following up Le Conte’s scheme, let us begin, then, to discuss briefly the scientific data, that is, considering the apparatus purely from the standpoint of physics and physiology.
A. Light.
Light.
I. Physical characters of the eye as an optical instrument.
If a ray of light passes through a small hole into a darkened room (pin-hole camera), an image is formed of the object or objects without. The condition of a good definition of the image is that “all the rays from each point on the object must be carried to its own point on the image.” If this hole be enlarged, this condition is impossible, and the light spreads over certain areas called diffusion areas or diffusion circles. In other words, widely divergent rays and contiguous rays become mixed. To admit more light a lens is used in the eye, and by the photographer, for although it is possible (by pin-hole camera) to take pictures without a lens, the light so admitted is necessarily so limited that the exposure needed is too long. The lens, however, helps us by admitting more light, and at the same time giving better definition, but it also introduces many disadvantages and sources of error. Now a theoretically perfect physical image has been described by physicists as being both bright and sharp in definition, but the theoretically perfect image does not exist; for, apart from other considerations, the lens which we use to get microscopic sharpness, cuts off light, and the sharper the image is rendered by stops, the less brightness do we get. Thus we see the lens introduces scores of errors as well as desirable qualities.
In the human and photographic lenses the chief faults are:—
Dispersion.
Dispersion. All refraction or bending of light by a lens is accompanied by dispersion. This error is corrected in opticians' lenses to a great extent. In the human eye, however, this fault is in some degree present, as can be proved by looking at a lighted street lamp through a violet glass, when a red flame will be seen surrounded by a bluish-violet halo. What, then, is the effect of dispersion on our theoretically perfect image? It is slight blurring of the sharpness of outline, since the size and position of the optical images thrown by the differently bent rays is not the same.