THE SENSE OF SIGHT.
The Perception of Light. agent that excites the terminations of the nerve-fibres in the retina is light. The sensation of light is produced in the brain by impulses reaching cer tain nerve-centres and coining along the optic nerves. These impulses are, in ordinary cir cumstances, sent along the optic nerves by the retina, and are communicated to the retina by the vibriaions of ether which are held to be the physical cause of light. But any excitement of the optic nerve, if it be passed on to the brain, will produce a sensation of light. Thus electrical stimulation of the optic nerve will do so, because it, equally with the usual stimulus of light, sets up changes in the brain cells, which occasion the sensation. Mechanical stimulation, of which the commonest form is "a blow on the eye," will also excite the nerve and produce sensations of light. It is the terminations of the nerve-fibres —the rods and cones, not the fibres of the nerve themselves, that are excited by light, for light falling directly on the optic nerve alone has no effect, while the feeblest glimmer of light will excite the retina and lead to a luminous impres sion.
The whole surface of the back of the eye is not, however, equally sensitive. There is, indeed, a spot, where the optic nerve enters the globe, completely insensitive to light. It is, therefore, called the "blind spot." Light falling upon it produces no stimulus. At this point there are no rods and cones, and in this fact is one reason for the belief that the rods and cones are the agents by whose aid the waves of light become transformed into the stimulus of a sen sation. A simple experiment proves this. Shut the left eye, and hold the thumb of each hand side by aide directly in front of the eye, with a good light falling upon them, and at the distance one would hold a newspaper in reading. Fix the right eye on the nail of the left thumb, and then move the right slowly away to aide. Though the eye is steadily regarding the left thumb both are seen, when the right is moved only an inch or so, but when the right thumb has been moved off several inches, the and joint disappears from view, though the shut hand is still visible, and when it has been moved a little further the whole right thumb is again visible to the eye, still fixed on the left thumb.
The explanation is that at a particular distance the rays of light from the end of the thumb fall on the blind spot, and give rise to no sensation, and when the hand is moved to one side or other of this place the rays fall on the retina on one side or other of the optic nerve entrance and so produce the sensation. If when the thumb has disappeared the hand be moved in any direction, forwards or backwards, the thumb will again come into view, for the rays will be made to fall on the retina. The same thing may be shown in another way. Shut the left eye and fix the right on the small letter a (Fig.176).
Then move the book near to or farther from the eye. In one position the large letter A disappears from view, in others both are visible.
The yellow spot (p. 449). directly in the centre of the back of the eye, is the most sensitive part of the retina to light. Objects are most distinctly seen when the eyes are so directed towards them that light from them falls on the yellow spot. In this spot cones are spe cially numerous, there are no fibres of the optic nerve, and the other layers of the retina are very thin. This is another reason for the be lief that the rods and cones are the true es sentials of the terminal organ of vision. In looking at any extended object the eyes are rapidly moved in various directions, so that its various parts are brought in line with the yellow spot. As a result of a fusion of all the different impressions received, which fusion is effected in the brain, the person judges of the appearance of the object as a whole. This grouping of impressions we are often unconscious of. It is done so rapidly and so habitually that we are apt to believe that we see with equal distinctness the whole of an extended object at once. In reading a printed page we know the eye moves so as to perceive one word after another in the printed line, and if we fix the eye on the centre of the line the ends will be indis tinct. It is because we move the eyes so rapidly, and because we learn to take notice only of the distinct impressions, due to rays of light falling on the yellow spot, that we are quite unconscious of the existence of a blind spot.