presented, otherwise indistinctness would prevail; and that the iris may more peremptorily regulate this most important principle, there is a dark colouring matter painted all around the interior of the eye, and thus the eye represents the principles used in the camera. The darker the matter the better, as preventing absorption of rays; so that we may say people with dark eyes can endure more glaring light, and, of course, admit more light into the eye. The great officers of the eye are,—sclerotica, giving the ball its form and firmness, conjoined to wonderful resilience, united by six muscles; the choroid provides warmth and nourishment; the transparent media refract light, and by causing convergence form an image accordant with the object, which image appears on the retina; the iris and pigment regulate the light and secure distinctness.
We have said thus much of objective vision, that is, we have spoken of light and of mechanism of the eye, and traced the means by which the external object is placed in the retina, and this may be said to be the materialism of our subject. But how all the sensible works of God and man become ideas and elements of the mind may be more difficult to prove. Yet we will follow in the beaten channel of authority, and endeavour to consider this other part of our subject,—viz., subjective seeing, or the means by which the retina surrenders its store of images and subjects to the mind or brain.
Perhaps the most simple form of explaining this more devious part of our subject, may be to say that when the image has been received by the retina, it has executed the mere mechanism of an optical instrument, and then the eye itself becomes the instrument of the mind, and the manner in which it is used by the mind may be called subjective seeing. For this there must be energy in the nerve of sense; that is, the retina and the optic nerve must
