Care of the Eyes

Shri Swami Sivananda

Introduction

The eye is the most wonderful, most delicate and most precious organ. It is the window of the soul. The presiding deity is Lord Surya or Sun God.

The eye gives beauty. It is the avenue of knowledge (Ayatana) in this world. It is the most valuable of all our senses.

A knowledge of the physiology, anatomy and hygiene of the eye, its wonderful functions, what the eye is, what it is made of, how, we see, how and why we lose our sight and how again it can be regained or preserved is highly useful.

Observe the rules for preserving sight. Practice the useful eye-exercise. Observe the rules of hygiene and health. Attend to your general health. Take regular exercise in the open air. Pray to the sun at sunrise. Lead a simple natural life. Take a balanced diet with vitamin efficiency. You will have wonderful eye-sight till end of your life.

Cataract

Your eye is covered by the cataract of ignorance. So you are blind. Walk steadily on the spiritual path in strict accordance with the teachings of your preceptor. The cataract will be removed by the remedy of your preceptor's teachings.

Chapter 1

Anatomy of the Eye

Hygiene of the Eye

The eye is a very important organ. It is the sense of perception. It is one of the five Jnana Indriyas. The external eye-ball is only an instrument to catch the vibrations of colour and light and send them to the centre in the brain. It is one of the five lamps in this body. It is a window in this body for the soul, the Proprietor of this body or the Indweller, The "will to see" or the "desire to see" has become the eye the organ of sight or perception. The presiding deity of the eye is Sun.

It consists of cornea which is a transparent carved window which receives the rays of light. The white part is the sclerotic coat. The space behind it is filed with a transparent watery fluid. There is a coloured curtain next to the cornea It is called Iris. It has a hole in the middle. The hole can automatically become large or small to receive more or 1ess light. The hole is called the pupil. In a bright light the pupil becomes small, in dim light it becomes large. The lens is behind the pupil. Behind the lens is the hollow of the eye which is filled with a vitreous body. Light passes through this vitreous humour and strikes the screen or retina behind the eye. Then the optic nerve takes the impression of the object seen from retina to the brain. If the optic nerve were cut out sight will be totally lost.

The knowledge of this sense universe is obtained chiefly through the avenues of the eye and ear. All colours and forms are centred in the eye, in the mind and Atma. It is really the brain that sees and interprets the impressions conveyed to the eye from the external universe. To be more correct it is the mind that really sees objects.

The eye is ever wandering and restless. It makes the mind also ever restless. Lust of the eye is very powerful. Steadiness of the eye is steadiness of the mind. Trataka, concentration, Upasana, worship, etc, make the mind steady.

The eye-balls are very safely placed in the bony sockets. They are well protected on all sides by the bony walls. Eyebrows and eye-lids prevent foreign bodies or dust from getting into the eyes. Mark how the all-merciful, all wise Mother Prakriti has given all sorts of protection from external injuries and foreign bodies. The muscles of the eye move the eye up, down and sideways.

Behind the eye-ball there is a soft pad of fat which serves as a cushion and lessens the chance of injury to the eye when it gets a blow. The disappearance of this pad of fat causes the sunken eye of very old and sick people.

A little tear gland (lachrymal gland) is placed in each socket of the eye-ball in the outer corner of the eye. A continual secretion of tears keeps the surface of the eye moist. A little duct open at the inner edge of each eye lid and carries the tears away into the nose. When dust enters the eyes they water involuntarily. This bathing of the eye with tears helps to wash the particles of dust into the corner of the eye. They act as little drains to keep perspiration out of the eye. They act as brushes to prevent dust from falling into the eyes.

The tear gland is easily irritated in women. Women shed tears profusely even when they are exited a bit. They are naturally weak. Tears are their weapons to accomplish their object. This is maya’s play in their lachrymal gland. Beware of false tears. Stand firm Do not yield.

The eye is like a camera. It is photographing the outside world on the retina. It receives the light rays from the sun. Just as images are printed in the plate of the camera, so also images of external objects are printed in the retinal plate of the eye.

If you are taking a photograph of something near at hand you have to focus your camera differently from the position for a distant landscape view. So also there is adjustment of the eye for seeing near objects. The shape of lens gets altered. The pupil also adjusts in its size. When you look at near objects the pupil is smaller then when you are looking at the distance.

The eyes should always be examined by a doctor in cases of headache or where there is any difficulty in reading the blackboard in the school. Squint is caused by straining of the eye.

Have sunlight treatment. You will have clear vision. You need not take recourse to glasses. Repeat the 12 names of Surya or Sun God in the early morning before sunrise. You will have good sight.

Sit in the sun. Close your eyes. Slowly move your bead a short from side to side. Let the sun shine directly on your closed eye-lids for ten to thirty minutes. Let the eye-balls also move with the movement of the head and not against.

Now turn the back to the sun or come in the shade. Do not open your eyes. Cover the eyes with palm of your hands for five or ten minutes. Avoid any pressure on the eyeball. This is palming. Have the sun treatment in the morning and evening 7 a.m. and 5 p.m. Let the exposure of the eye to the sun be gradual. When the eye gets accustomed to the light, you can gradually have strong light.

Do not read in a bad light or in a light that flickers. There is a great strain on the eyes. It will produce eye trouble. The light should come from behind and above or from the left side. Do not read or work in the petromax light. It will spoil your eye-sight. If you are forced to work, place the light behind your head. Have a green shade. Too much reading, and close work such as fine sewing is not good for the eyes.

Good eye-sight is very necessary for success in life. The eye is the most sensitive organ. Do not apply anything and everything which laymen suggest for diseases of the eye. The eye will be spoiled. Always consult an eye specialist. Do not neglect early treatment in granular lids. The granules should be frequently touched with copper sulphate. Do not remain in the smoke.

The commonest causes of disease of the eye and deformative eye sight are overuse and straining of the eye by reading in a bad light, reading books printed in too small type or glazed paper, glare, foreign bodies getting into the eye, draughts of cold air, lack of proper food, fresh air and general bad health. Do not read the book when lying down in bed.

Use Brahmi Amalak Oil. It will cool and refresh the tired eyes. If there is a foreign body under the eye-lids a boric eye wash. Open and shut the eyes within the eye cup or open the eye under clean water. Put a drop of cashew oil in the eye at night. It will come off by itself in the early morning. If these methods fail, consult a doctor.

The eye is a restless organ. You must give it a proper rest. Restless eyes produce restlessness of the mind. Sit calmly, close the eyes and do your japa and meditation. You can give ample rest to the eyes. Do this two or three times a day, morning, evening and night.

He who dwells within the eyes, who is within the eye does not know, but whose body the eye is, who rules the eye from within is thy inner Self, Atma, Immortal, Amrit. Atma is the eye of the eye. Realise Him through the inner eye of intuition and be ever blissful and free.

Organ of Sight

The eye teaches you a great deal about the world in which you live. It receives the light rays which are continually streaming form the sun. The eye ball is an organ sensitive to light. It is suspended in its bony cavity in the skull. The cavity protects the ball on all sides except at the skull. The cavity protects the ball on all sides except at the front where it is guarded by the eye lids. The eye-ball is attached behind the optic nerve.

Protective Appendages of the Eye

The eye lids are folds of skin containing thin plates of cartilage. They are fringed at the edges with hairs, the eye-lashes and contain a series of small glands called meibomian glands. The secretion of these glands is provided to prevent adhesion of the eyelids The eyelids are covered internally by a mucous membrane, the conjunctiva which is reflected from them over the globe of the eye.

The eye lashes are arranged in a double or triple row at the margin of the lids. The upper lid is attached to a small muscle which is called the elevator of the upper lid (levator palpebrae superioris). The muscle orbicularis palpebrarum is sphincter around both lids. These muscles shut the eyelids. It is the direct antagonist of the elevator of the upper lid.

The slit between the edges of the lids is called the palpebral fissure. The size of this fissure causes the appearance of the large and small eyes but not the size of the lobe it self. The size of the lobe varies but little.

Conjunctiva is a delicate mucous membrane which lines the interior of the eye-lids and the front of the eye-ball. Inflammation of the conjunctiva is called conjunctivitis or sore-eyes.

The eye-lids are provided for the protection to the eye They are movable shades which by their closure excludes light particles of dust and other injurious substances.

Eye-brows

They are composed of two arched eminences of the thickened skin. They are connected with three muscles which by their action control to a small extent the amount of light admitted into the eye. They are furnished with numerous short thick hairs.

Lachrymal Apparatus

This apparatus consists of (1) the lachrymal gland (2) canaliculi (3) lachrymal sac, (4) nasal duct. The lachryma gland secretes tears. It is placed at the upper and outer angle of the orbit. Seven to twelve minute ducts lead from the gland to the surface of the conjunctiva of the upper lid. The secretion (tears) is usually just enough to keep the eye moist. It passes over the surface of the eye-ball and is sucked into the tiny canaliculi through the punctae and is conveyed into the lachrymal sac which is the upper dilated portion of the nasal duct.

The nasal duct is a membranous canal about three quarters of an inch in length. It extends from the lachrymal sac to the inferior meatus of the nose into which it opens by a slightly expended orifice or opening.

The tears consist of water containing a little salt and albumin. They are ordinarily carried away as fast as in forms, but under certain circumstances, as when the conjunctivs is irritated by pungent vapours, or when painful emotions arise in the mind, the secretion of the lachrymal gland (tears) exceeds the drainage power of the lachrymal duct. The fluid accumulates between the lids and overflows in the form of tear. It runs down the cheeks.

The Muscles of the Eye-ball

The muscles which move the eye-ball are six in number. Four muscles are straight (recti) and two oblique. The recti muscles are the superior rectus, inferior rectus, the external rectus and the internal rectus. The eye-ball is completely embedded in fat behind and laterally. These muscles turn the eye-ball as on a cushion. The superior rectus turns the eye upwards, the inferior down wards, the external outwards, and the internal inwards. The action of the oblique muscles is complicated. They roll the eye-ball on its axis and pull it a little forward and inward. The eye-balls can be turned and moved in any desired position or direction. If the eye-balls are not endowed with such a movement, you will have to turn the whole head frequently in your vision of object it will be a great nuisance, trouble and difficulty. Mark here the grace of the Lord and His Wisdom in creation.

The Eye-ball

The visual apparatus is the eye with its accessory organs and the optic nerve. Eyebrows, eye-lids, lachrymal apparatus, and muscles of the eye-ball are the accessory organs of the eye. The eye-ball is really a minute camera which is constantly photographing the outside world on to the retina It is covered with a strong white coat and is filled with a transparent jelly. There is a transparent curved window in front through the light rays enter. Behind this, there is the lent of the camera. It if a little, clear, oval body, shaped like two watch glasses placed together the edges touching Iris, coloured curtain is stretched between the lens and the window. It has a hole in the middle known as the pupil. The iris contains a delicate ring of muscle fibres which make the opening id the curtain smaller when they contract. The Iris is the diaphragm of the camera.

The eye ball is contained in a cavity, the orbits. Seven bones assist in the formation of each orbit namely frontal malar, maxilla, palate, ethmoid, sphenoid, and lachrymal Each orbit is nearly two inches in depth. It is padded with fat and lined with a membranous capsule - the capsule of tenon Which is a serous sac. Thus the eye-ball is isolated from surrounding structures. It can freely move without friction. The eye-ball is protected from injury by the body wall, the eye-lids and the eye-lashes.

The eyeball is composed of three coats or tunics and contains three refracting media or humours. The three coats are (1) sclera and cornea (2) choroid, ciliary processes and iris and (3) retina. The three refracting media are (1) aqueous humour (2) crystalline lens and capsule and (3) vitreous humour.

Six striated muscles connect the eye-ball with the wall of the cavity or orbit, four of these muscles are straight on recti-muscles and are called superior, inferior and external and internal recti-muscles. There are two slanting muscles the superior and inferior oblique muscles. The eye be directed or rotated to any direction and at any angle with the aid of these six muscles. The movement of eye-ball is really a very complicated one.

The optic nerve has its root in the brain. It leaves the skull by a hole at the back of the orbit and enters the back of the globe of the eye. It spreads out into a very delicate membrane which lines the hinder two thirds of the globe or the eye-ball and is termed the retina. This retina is connected with sensory nervous fibres. It gives rise to the sensation of light.

Sclera

The sclera covers the posterior five-sixths of the eye-ball. It is opaque, white and smooth. Behind it is pierced by the optic nerve. It is a thick, fibrous outer coat of the eye ball. It serves to protect the delicate structures contained within it. It is the protective the choroid, the vascular or nutritive and the retina is the visual or perceptive layer of the eye-ball.

Cornea

This covers the anterior sixth of the eye-ball. It is directly continuous with the sclera or sclerotic coat of the eyeball. It is composed of fibrous tissue. This is the "window of the eye". It is perfectly transparent but appears dark, as dark internal structures are seen through it. Just beyond the edge of the cornea there is a thin shining membrane, called the conjunctiva. This is attached to the eye-ball all round. This is kept well-lubricated by the secretions of the eye. It gives the eye-ball its peculiar shinning and glimmer.

Choroid

The eye of a man appears black in the centre with a variously coloured ring around the centre. The ring is variously coloured. It is circular curtain attached to the edge of the cornea all around. There is a hole in the centre which appears dark. The curtain is called iris. The hole is called the pupil. The colour of the iris may be black, brown or blue. The colour is due to the varying amount and distribution of granules of black pigment in it.

At the edge of the cornea the iris is continuousness all round which a second coat which lines the border three fourths of the eye. The coat is called the choroids. It is loosely attached to the inner surface of the scoloretic up to the edge of the cornea where it leaves the outer edge and just cuts across the edge forming the iris. The choroids is much thinner than the schequid. It is abundantly supplied with blood vessels. Its inner surface is black as it is lined by a layer of cells filled with granules of black pigment. The back of the iris also covered by a similar layer of cells filled granules of black pigment. The choroid is thrown into a number of folds arranged in a radiating manner all round just before it becomes continuous with the iris. These folds are known as ciliary processes. They, like the choroid, are covered by a layer of black pigment cells.

Iris

The iris is like the diaphragm of a camera. The pupil is like the aperture or hole of the diaphragm. The size of the pupil is variable. It can be regulated by the muscles of the iris or the diaphragm. In shade the pupil enlarges by taking in more light in the eye through the enlarged hole.

In direct light of the sun the pupil contracts to a pinhole so as to shut but all unnecessary light.

Iris (rainbow) is a coloured, fibro-muscular curtain hanging in front of the lens and behind the cornea. It hangs free in the interior of the eye-ball. The light is admitted into the eye-chamber through the pupil. The sphincter muscle is the contractor of the pupil. Another set of muscle is the dilator of the pupil. The posterior surface of the iris is covered by a thick layer of pigment cells designed to darken the curtain and prevent the entrance of light. The anterior surface of the iris is also covered with pigment cells. It is particularly these latter which produce the beautiful sheen in the iris. The different colours of the eyes are chiefly due to the amount and not to the amount and not to the colour of the pigment deposited.

Iris cuts off the rays which would otherwise pass through the outer parts of the crystalline lens and be brought to a focus too soon.

Pupil

If you look at the pupils of any one's eyes when the sun is shining brightly, you will notice that they are small. The curtain (iris) has contracted and has shut a great many light rays. But if you shade the eyes or if the person turns away the light the pupils will at once become, larger. The eyes of the animals are glowing like lamps in the dark, because some fays of light are reflected back from the retina. In animals the pupils are very big in the dark to enable them to catch as many light rays as possible.

When you look at near objects the pupil is smaller then when you look at an object which is at a distance, because it is easier to focus clearly for near objects when the external rays reflected from them are cut off.

The pupil is larger when the light is dim and also when you look at a distant object. It is smaller when the light is strong and when you look at near objects. The sphincter muscle that is placed near the margin of the pupil contracts and the pupil becomes smaller. When the muscle relaxes the pupil becomes again larger.

Lens

Transparent, glass like double convex body is situated at the back of the pupil immediately behind the iris. This is the crysgeline lens. It is kept in place by a sheet or transparent tissue called the subpensory ligament. It passes from its edge and attaches it all round to the ciliary process. It forms a ring all round the lens. It is composed of a jelly like substance placed in layers like those of onions. The layers cavity between the crystalline lens in front and the retina behind is filled with a matter called vitrous humour, which is a clear semi-fluid substance. It supports the retina which lies upon its surface and preserves the spheroidal shape of the eye ball. The space between the cornea and the iris, is called the anterior chamber of the eye, it is filled with a fluid called the acqueous humour which is thin and watery.

Its refractive power is much greater than that of a aqueous or vitreous humour. It acts by virtue of its double convex from as a converging lens, bringing parallel or diverging rays to a focus on the posterior surface of the retina. The function of the crystalline lens is to bring to a focus all the rays of light emanating from each separate point in the objects seen, so that all the light from each point falls on and stimulates a corresponding point on the retina. If the eye consisted only of a sensitive retina impressions of light could be received, but the form of objects would not be distinguished.

Perception of the figure of external objects depends upon the action of the crystalline lens in converging all the rays emanating from a given point to a focus on the retina. The lens is enclosed by a transparent membrane called the capsule of the lens The suspensory ligament is attached to this capsule.

Retina

The retina is the seeing part of the eye. It is spread out in the form of a delicate membrane on the inner surface of the eye-ball. It is connected with the brain by a large nerve which enters the back of the eye-ball.

At the back behind the lens is the retina which is the sensitive portion of the eye. It is almost entirely composed of the nerve terminations of the optic nerve. This is the innermost coat of the eye-ball. It is the most essential part of the organ of sight, as it is the only one directly sensitive to light.

The retina lines the interior of the eye. It is placed between the between the etoroid and vitreous humour. It consists of eight layers. The eighth layer or layer of nerve-fibres is the internal layer. The seventh layer is the layer of nerve cells. The first layer or the layer of rods and cones is the external layer.

The fibress of the optic nerve pierce the sclera and choroid at the back of the eye and spread out and form right or innermost layer of the retina. The fibres pass peripherally through the other layers and terminate in layer of rods and cones. Rays of light do not produce any effect^ the optic nerve without the intervention of the rods and cones.

Rods and Cones

Light exeites rod-like processes of cells in the retina and impressions pass from these to the optic nerves. These lie in the retina in a definite layer, called the layer of rods and cones. There is a connection between the fibres of the optic nerve and the rods and cones. The rods and cones are the essential nervous elements of the retina.

The layer of rods and cones is at the outer surface oft' retina, that is the surface next to the choroid coat, while the layer of nerve fibres and of nerve cells is next to the vitreous humour. The light which comes through the vitreous humour has to pass through the retina, itself, before it excite the rods and cones. Therefore the whole retina is transparent.

Vision or Seeing

The rays of light excite in the optic nerve impulses which reach the brain and produce the sensation of sight. Ill the act of seeing the rays of light fall on the eyeball, pass through the transparent portion of the cornea penetrate the aperture of the pupil. The pupils cut off side rays in order to give a clear image.

The aperture is also regulated by the amount of light required. Light from the object then enters the lens, passes out of it and strikes the retina. The lens is biconvex. The image formed on the retina is a reversed image of the object. The actual seeing is done by the visual centre in the brain. The brain corrects the inverted image to an upright one. The lens of the eye focuses the rays of light for us on the retina. It is placed at just such a distance from the retina that all the rays of light coming from distant objects are focused there.

Visual Sensation

The impression made by light lasts for a certain time after the light is taken off. This is about one eighth of a second. If two flashes follow each other at a less interval of time than this, their impressions catch each other up and produce one sensation only. The eye sees a continuous flash. That is the reason why the spokes of a rotating wheel are not seen separately at night gives the idea of a circle of fire. The projection of cinema pictures is based upon this. Separate pictures are flashed before the eyes very quickly. The eyes join the separate units and get a moving picture.

The sensibility of the retina is easily tired. If you look at a bright light for some time, and then look at a sheet of paper a dark spot or dark image of the bright light is seen on the paper. The reason is this. The retina is in a state of fatigue after looking at the bright light. And so the light rays from the paper cannot excite the sensation of light.

Ordinary light or white light can be split up into a number of rays which fall on the retina and produce the different sensations called colours. The retina may similarly be fatigued for one colour only, that is for one portion only of white light. If you look at a piece of red paper lying on a sheet of white paper steadily for some time and if the red paper is red, moved suddenly you will see a green patch where the red patch lay. The reason is this. The part of the retina on what the image of the red patch fell got tired of red rays. After the red patch was removed, there was no effect on the retina by the rays which were present in the white light, now coming from the same spot. It is as if the white light coming from the spot contained no red rays. But if you take the red rays out of white light the result is green. Red and green light milted together make white light. Red and green are complementary colours. When the eye is fatigued for one the other is seen. The same is the case with yellow and blue.

Some suffer from colour blindness. They are not able to see the differences between some colours. They are not able to see red and green as distinct colours. They cannot tell tot difference between red and green objects except by their shade brightness. This is a very important matter in the selection of engine drivers and sailors. A few persons are totally colour blind. Everything appears to be of the same colour to them.

Firm pressure on eye produces a height image or light (phosphene). A blow on the eye or a fall on the head produces flashes of light. In these instances then there is no falling of light on the retina. These effects have their origin in the retina itself They appear to you at sensations of light.

Colour Vision

When you look at a rainbow you behold seven distinct kinds of colour sensations. They are VIBGYOR (violet, indigo, blue, green, yellow, orange and red). When ordinary white light it passed through a prism and then allowed to fall into the eye. You experience the same seven coloured sensations. These are called colour of the spectrum. Bach colour is characterised by certain qualities as in the case of sounds. These are: (i) Hue such as red, green, blue, yellow, etc. This depends on the wave-length of the ethereal vibrations. This corresponds to the pitch of sound, (ii) Intensity or brightness. This depends on the amount of light which fall on the retina in a given time. This corresponds to the loudness of a sound, (iii) Saturation or the amount of ad mixture with white light. A colour is pale if it mixed, with much white, deep, rich or full if highly saturated i.e. unmixed with white.

The colours of objects depend on the power they posses of absorbing some of the constituents of ordinary while light and allowing others to pass or to be reflected. Thus a piece of glass is blue if it allows the blue rays to pass to the eye and steps the other. The colour of an opaque blue object is due to an absorption of the spectral colours other then blue by the superficial layer of the objects and the reflection of the un-absorbed blue rays from its internal parts.

There are several pairs of colours such as red and green orange and blue, yellow and blue, greenish yellow and violet which when mixed in the right proportions give rise to the sensation.

These are called as complementary colours. Every colour has some other colour which is complementary to it. If we mix the colours in threes it becomes still more easy to produce white. An excellent white colour is obtained if we mix red green and blue in proper proportion and intensity Also these three colours and their mixtures produce all the several kinds of colour-sensation which we get from a spectrum. By suitable mixture of these colours with white or black we can produce the other colours which we see in natural objects around us but which are wanting in the spectrum. Thus purple can be made at once by mixing red and blue. Therefore these three colours are regarded as primary colours.

Accommodation

The lens like all ordinary lenses forms the image of the object at its locus. The place where the image is formed behind the lens depends upon the distance of the object and also upon the curvature of the lens. In the case of the eye the place of image is fixed at a definite place. The image must fall on the retina. The curvature of the lens is varied by means of the ligament attached to the lens in order to accomplish this. The lens becomes flat for distant object it becomes more convex for nearer object. The power of the lens to adapt its curvature according to the distance of the object is termed accommodation.

When you take a photograph of something near at hand you have to focus your camera differently from the position for a distant landscape view. In a camera this can be done by altering the lens or by altering the distance of the screen. In the eye the distance between the lens and the retina remains the same, but the shape of the lens is changed.

The lens is elastic. If its surface is made flatter by pressure, it recovers its original curvature and shape when the pressure is relieved.

In accommodation for near objects, the lens becomes more convex and the pupil of the eye likewise contracts. The convexity is brought about by the action of the ciliary muscle and is always more or less fatiguing. The accommodation for distant objects is a passive condition, the convexity of the lens being unaltered and the pupil of the eye dilated and it is on this account that the eye rests for on indefinite time upon remote objects with out fatigue.

Blind Spot

The optic nerve pierces the eye-ball not exactly at its most posterior point but a little to the inner side of it nearer the nose. This point where the optic nerve enters is called the ad spot. There are no rods and cones at this spot. Light ailing on this spot produces no effect. It is therefore called the blind spot. This is a proof that the sensation of light cannot arise without either rods or cones.

Yellow Spot

There is one point of the retina that is of great importance. This is the yellow spot (macula lutea) It is the epicentre of the retina. It is situated about 1/12 inch to the side of the exit of the optic nerve. In its centre is a tiny fovea centralis. This is the centre of direct vision. This is the part of the retina which is always turned towards the object looked at. From this point the sensitiveness of the retina grows less and less in all directions. At the point for centralis no fibres of the optic nerve are found. But then a great increase in the number of cones as well as in the size. It is in the yellow spot that vision is most distinct. The central part of this yellow spot all the parts of the retina except the layer of rods and cones are extremely thin. Light can here most easily pass to the cones. When you wish see a thing distinctly you look straight at it so that its image falls on the yellow spot. You see the other thing around, not so distinctly, as their images are falling on the other parts of the retina.