The sense cells of the retina are affected by light, setting up impulses which pass along the optic nerve to the brain, where the sensation of light is experienced. All other parts of the eyeball are concerned with causing images of the objects seen to fall on the retina, so that we experience a picture of the object seen, not just a mass of spots of light. All transparent parts transmit light. The pigmented parts restrict its rays, that in the irisoff all those which do not pass through the pupil, which, being, the aperture of the eye, acts like the diaphragm of a camera, reducing the amount of light falling on the retina. Pigment at the back of the retina and in the choroid absorbs light and so prevents reflection. The cornea, aqueous humour, lens and vitreous humour all act as refractive media causing the images falling on the retina to be formed in detail. The images formed are inverted on the retina, but in the brain the positions of the images are rectified.
Experiment 72—To show that the Image on the Retina is Inverted
A hole is pierced in a piece of paper by means of a pin. The paper is held about ½ in. from one eye. While closing the other eye, the light coming from a window is viewed through the hole in the paper. All objects viewed through the hole appear in their rightful. The head of the pin is then slowly pushed upwards between the paper and the eye so as to obstruct the light coming through the hole. The shadoio of the pin now falls on the retina and not its image. The shadow appears to descend dowmcards. The brain, accustomed to invert all images ’seen ’by the retina, behaves as usual and inverts the shadow which is obviously falling on the lower part of the retina and not on the upper as it is apparently ’seen. ’
Experiment 73—To demonstrate the Blind Spot
A dot and a cross are made on a sheet of paper about 6 in. away from each other. The left eye is closed and the paper is held at arm ’s length with the right eye directed to the cross and kept on to it, while the paper is moved slowly nearer and nearer to the eye. The image of the spot falls on the retina on the side nearest the nose. As the paper approaches the eye, its image traverses the retina and eventually falls on the blind spot, when it is no longer seen. As the paper gets nearer the eye still, the spot comes into your view again as the image moves farther on and passes beyond the blind spot.
Now objects at different distances from the eye do not all form images in focus at the same time, but the image of any one object can be focused either by altering the distance of the lens from the retina or by altering the shape of the lens. These methods are used in the frog and rabbit respectively. This focusing activity is called accommodation.
Since the sense cells of the retina are normally affected only by light, it follows that no animal can see in the dark. Its eyesight may, however, be so sensitive that it can see effectively in a very dim light, e.g. the owl, or may have a very keen sense of hearing, touch or smell, e.g. bats, which have poor eyesight yet are usually nocturnal animals.
The Eye of the Frog
In the frog the angle of vision is wide, there are no true eyelids, but there is a transparent third eyelid for protecting the delicate conjunctiva when the animal is under water. The cornea is slightly flattened. The iris is a gold colour. The pupil is relatively large, so that it is very doubtful whether the animal sees anything in detail. Judging by its responses, it seems to be affected only by moving objects, but its tongue-flicking reflex is extremely accurate. The eye-lens is very thick and, to focus distant objects, is moved towards the retina.
The Eye of the Rabbit and Man
Above and below each eyeball is a slightly muscular fold of skin, an eyelid which is fringed with eyelashes, long stiff hairs for preventing objects such as insects from damaging the conjunctiva and cornea, since the conjunctiva is extremely thin. Above each eyeball at its outer edge is a tear gland which pours its secretion along tiny ducts over the front of the eye to wash away any particles which lodge there. It drains away at the inner corner of the eye and goes down the nasal passage—hence the sniffing which accompanies crying.
The Accommodation Reflex
The lens is held in place by a ring-shaped suspensory ligament which is surrounded by a circular ciliary muscle in the ciliary processes of the choroid. When not subjected to any strain the lens is ’fat. ’ When the ciliary muscle is relaxed, the suspensory ligament exerts a pull on the lens, causing it to flatten and become thinner. This is the condition in the resting eye, which is thereby focused for objects at infinity. To effect accommodation for nearer objects the ciliary muscle contracts ; this pulls the margin of the choroid nearer to the lens. This reduces the strain on the lens, which, being elastic, becomes thicker, the anterior surface bulging forwards until the rays are brought to a focus on the retina. At the same time the pupil narrows. The whole process is an unconscious reflex action.
There is a limit to the thickening of the lens due to its elasticity. This is easily found by gradually moving a vertical finger nearer and nearer to the eye. Its details cannot be detected when less than 9 cm. away. A sense of strain is experienced on forcing the eye to bring the finger into focus—the strain due to the contraction of the ciliary muscle. Hence the danger of holding print too near to the eyes when light is dim or the print is too small.
Another purely unconscious reflex of the eyes is that which swivels the eyes inwards when near objects are being viewed. This is important, as similar areas of the two retinas in our eyes send impulses to the same parts of the two cerebral hemispheres of the brain, so that we see only one set of objects. The swivelling enables us to judge the distance and size of an object, whilst the two coincident sensations enable us to detect details that we might otherwise miss.
The Structure of the Retina
The light sensitive cells are of two types : the rods and cones. They occur in the innermost region of the retina and are connected with the brain via relaying neurones by the optic nerve.
In the yellow spot cones only occur. The rods are concerned with vision in dim light, while the cones are used in bright light and probably for colour vision. It is believed that there are three types of cone. One type is stimulated mainly by the light rays of the red end of the spectrum, a second by the green middle region and the third by those of the blue-violet end. If one or other of these types is missing or fail to function, inability to distinguish corresponding regions of the spectrum results and colour blindness is said to occur. The commonest type of colour blindness is the inability to distinguish between red and green.
In dim light a pigment called visual purple accumulates in the rods. These then become more sensitive to light and the eye is said to be dark-adapted. The pigment takes a little time to accumulate in the rods and that is why some few minutes elapse on going into an almost-darkened room before objects can be seen. On coming out of the dark room into bright daylight, the pigment becomes bleached. Some people are unable to produce visual purple and are said to suffer from night-blindness. In normal people an adequate supply of vitamin A is needed to form visual purple.