The Eye

- our window to the outside

We read in the Bible that “… the eye is not satisfied with seeing, …”  (Ecclesiastes 1:8). The eye is actually one of our most important sense organs, since well over half of all the information we take in about our surroundings comes to us by way of our eyes. Through being able to perceive incident light, we can read letters, newspapers, and books; marvel at the colours of a blossom, the perspectives of a landscape, the beauty of a dress, or the artistic appeal of a painting. But especially, we can see our loved ones and others whom we encounter in our daily lives. The German word for face is Gesicht which means the same as vision or sight. The English (and French) word visage, also meaning face, similarly refers to seeing (from the Latin videre = to see).

Physiologically speaking, seventy percent of all our sense receptors are located in the eyes. In reality, we evaluate and understand our world mostly from being able to see it. That’s why, despite all their differences, all human languages are rich in visual imagery. Figures of speech and proverbs, though conveying abstract meanings, are often easily visualised, like: “Up to one’s neck in debt”; “Carrying your heart on your sleeve”; “A rolling stone gathers no moss”.

In the Bible, the Creator commanded on the very first day: “Let there be light!” Our visual sense was thus provided for right from the beginning. When He reviewed His creative works, we read five times: “God saw that it was good” In reviewing all He had made at the end of the six creation days, we again find His evaluation based on vision: “And God saw every thing that he had made, and, behold, it was very good.”  (Genesis 1:31).

Having established the importance of vision, we now turn to the actual organ of sight.

General features of the eye: Visible light comprises electromagnetic radiation with wavelengths between 400 (violet) and 750 (red) nanometres (1 nm = 10-9 m = one millionth of a millimetre). For the purpose of forming an image, the incident light rays must be bent (refracted) and focused sharply on the retina. The cornea handles most of the refraction and the lens subsequently focuses images at various distances by varying its curvature. Through this ingeniously devised ability to change its shape, the focal length of the lens can vary between 69.9 mm and 40.4 mm. This is why, unlike the best products of the optical industry, we can manage with only one lens.

The iris acts like the diaphragm of a camera. There are two opposing sets of muscles which regulate the size of the aperture (the pupil) according to the brightness of the light. The shape of the eye is maintained by the vitreous body, and the pressure in a fluid called the aqueous humour which fills the anterior and posterior chambers. This pressure depends on a balance between the production of this fluid and its outflow. The cornea is lubricated, and protected against drying out, by the tear ducts and the movements of the eyelids.

Of all our sense organs, our eyes have the greatest range of detection sensitivity, as well as the greatest adaptability. They have their own machinery of movement, through special muscles which enable vision to be directed towards a target. The two-dimensional image on the retina requires massive parallel processing in the subsequent network of nerve fibres.

Structure of the eye: The eye can be divided functionally into two parts, namely the physical dioptric mechanism (Greek: dioptra = something through which one looks) which handles incident light, and the receptor area of the retina where the light triggers processes in nerve cells. The dioptric mechanism produces a miniaturised, upside down image. To obtain a sharp image requires an exact “fine tuning” between the refractory (light-bending) properties of the optical medium and the dimensions of the eye. A deviation in the latter as small as 0.1 mm is enough to cause faulty vision, requiring correction by spectacles.

The cornea’s main task is to protect the delicate components of the eye against damage by foreign bodies. The iris is located between the cornea and the lens, and its function is to control the amount of incident light, in the same way as the diaphragm of a camera. The lens focuses the incoming light rays on to the retina (Latin: rete = net), where the actual process of perception begins. The photo-receptors (the rods and cones) convert the incident ’optical signals’ into chemical (and subsequently into electrical) signals. These electrical signals then travel to the brain along the optic nerve. There are no photo-receptor cells at the point where the optic nerve leaves the retina, and this is known as the “blind spot”. Another important feature of the retina is the so-called “yellow spot” (the macula lutea). In the middle of this is the fovea centralis, the point where visual acuity is at a maximum. There are no rods in this position, only cones, which are connected in a special way to the relevant nerve cells. When you focus your attention on a certain object, your head and eyes automatically move in such a way as to let its image fall on the fovea for greatest sharpness.

The retina: The back of the eye can be observed through the pupil using an ophthalmoscope. The retina, with the blood vessels supplying its inner layers, can be seen, as well as the blind spot and the yellow spot.

The retina plays a key role in visual perception. This thin (only 0.2 mm) layer of nerve tissue lines the inside of the eyeball. It contains the photoreceptor (light-sensitive) cells and four types of nerve cells, as well as structural cells and epithelial pigment cells (Latin: pigmentum = dye; Greek: epithel = outer layer of the skin). The two kinds of photo-cells are called rods and cones because of their shape. These microscopically small light detectors, which contain the various visual pigments, are masterpieces of technological efficiency.  Each eye has about 110 million rods and 6 million cones. They form a laterally interconnected network and are connected “vertically”, by means of so-called bipolar cells, to the one million ganglion cells. These collect all the optical signals received by the retina, determine the direction of flow of these signals, and transmit them to the brain through the optic nerve. This bundle of more than one million nerve fibres, each well “insulated” from the others, is about 2 mm thick. Present-day communications experts using glass fibre technology can only dream of a “cable” of this kind.

One single square millimetre of the retina contains approximately 400,000 optical sensors. To get some idea of such a large number, imagine a sphere, on the surface of which circles are drawn, the size of tennis balls. These circles are separated from each other by the same distance as their diameter. In order to accommodate 400,000 such circles, the sphere must have a diameter of 52 metres, nearly three times as large as the hot air balloons used for advertising promotions.

The photo-receptors: The rods and cones not only differ in shape, but also in function. The rods are cylindrical, while the cones are smaller and have a tapered form. In the case of low illumination as at night, the rods enable us to distinguish between brightness and darkness. They are so sensitive that the absorption of a single photon results in a measurable electrical signal. This high sensitivity is achieved through having a long time lag (about 0.3 seconds) between the absorption of a photon and the emission of the electric signal, allowing a complex amplification process to take place.

The cones operate much faster; their time lag is only 0.075 seconds, but they are much less sensitive than the rods, and only function optimally in daylight. There are three types of cones, distinguished by their absorption maxima, each being most sensitive for, respectively, red light (having a wave length of approximately 705 nm), green light (520 nm), and blue light (450 nm). By comparing the messages received from the different cones, the ganglia identify the colours actually observed.

We would expect the light receptors to be on the side of the retina exposed to the incident light, but, amazingly, this is not the case. The light must first pass through another layer of the retina. That is why it has been said that our eyes have “inverted wiring”, an arrangement which nevertheless works brilliantly.

The light sensitive cells act like interpreters, translating the impulses of light into the language of the nervous system. Another way of putting it is that a photo-receptor cell is basically a counter which counts the number of incident light quanta (photons). Its sensitivity ranges over five powers of ten, and it is able to adapt to the brightness of the prevailing light conditions by altering its sensitivity. For example, in response to bright light, it can reduce its sensitivity 100,000 times!

Sensitivity: We are blessed with extremely sensitive sense organs. Furthermore, the Creator solved a universal technical problem. Whenever a radio receiver is set for maximum sensitivity, it becomes noisy. This hissing sound is caused by the irregular thermal (heat) motions of electrons in the resistors. It can be eliminated by cooling all the components to a temperature far below freezing. But this is impractical, and is technically impossible for signals having the same strength as the (statistical) noise. A certain trick helps – transmit the signal on two separate channels and subsequently combine them. In this way the random noise fluctuations in each partially cancel each other out, resulting in an appreciable reduction in noise.

This method is also employed in the eye. In sensory organs and nerve cells, “noise” is not so much a result of fluctuations in electron density, but is caused by fluctuations in the voltage on the interfaces between sensory and nerve cells. The Creator made our optic cells as sensitive as physically possible. As mentioned, one single light quantum (photon), the smallest physical unit of light, is sufficient to cause an electric impulse in an optic cell. Any possible illusion which might be caused by “noise”, is eliminated as follows:

Several hundred rods, the most highly sensitive cells, are connected to only one nerve cell. These special nerve cells only transmit an impulse if a sufficiently strong signal has been received from at least four or five optic cells within a certain time period, about 0.02 seconds. This means that the individual optic cells are as sensitive as at all physically possible, but the nervous system only transmits signals when several impulses arrive more or less at the same moment, after a certain summation period. Thus the maximum possible sensitivity only comes into play when the light stimulus arises virtually simultaneously from receptor cells spread over a sizeable area, and not from just a single point. Random “noise“ fluctuations would arise at different times in each cell, so are never transmitted.

Visual acuity: Visual acuity (sharpness), the ability to resolve objects, is very important in the assessment of vision. Under good illumination a normal eye can distinguish between two points if the incident light rays make an angle of 1 minute (1’ = 1/60 degree).

Adaptation (Latin: adaptio = adjustment, especially that of sensory organs to the prevailing conditions): Our eyes are able to process bright and dim light over a wide range. At night we can observe dim stars, and we can also adapt to the glaring intensity of bright sunlight reflected from snow and ice. This amazing adaptability of the eye spans an immense range – a factor of 1 to 1 million million!

Colour perception: We would have missed something wonderful if we could not see colours! Colours may bring joy, and can even affect our moods. They contribute to happiness and affect our state of mind. Colours fascinate all of us, not just artists and fashion designers.

Colours can be characterised by three aspects, namely hue, brightness, and saturation (= the degree of admixture with white). Our eyes can distinguish 300 different hues or shades of colour, and if, in addition, the brightness and saturation are varied as well, several million possible colour values can be distinguished. The brightness of a colour is determined by the strength of illumination, and the saturation.

In our eyes it is only the cones which can detect colours. The chemical involved is called rhodopsin (Greek rhodon = a rose), or visual purple. It consists of protein molecules (comprising approximately 350 amino acids), including the so-called retinal which colours the rhodopsin. Retinal also makes the rhodopsin sensitive to light, similar to the way a detonator makes a cartridge sensitive to being struck by a firing pin. The rhodopsin of a cone cannot absorb all the light quanta (photons) which strike it; it “selects” quanta of a certain size (wavelength). It will capture most or all of such quanta, but will also capture one out of ten to one out of fifty of those which are exactly double or half the preferred size. However, each photon captured has the same effect, regardless of the wavelength. There are three types of cones, each preferring a specific, optimal quantum size. They are known respectively as red-, green-, or blue-sensitive cones, according to the optical pigments and the preferred quantum size (wavelength of the incoming photons of light). But all this still does not mean that we can see colours – it only provides the necessary preconditions. The sensation of colour only arises in the brain after a computational comparison of the excitation of the three types of cones. There are about 100 million optic cells in the retina, but only one million optic nerve fibres lead from them. This means that many optic cells are interconnected in a complex fashion. The optic nerve transmits image information to various parts of the brain in the form of electrical pulses. A small number of fibres lead directly to the mid-brain, but most of them converge on a switchboard which serves the primary vision centre in the rear of the brain.

The images formed on both retinas are upside down and also left-right inverted. But an astounding fact is that the optic nerves from both eyes split up and cross each other in such a way that the left halves of the images of both eyes are received by the right half of the brain, and the right halves end up in the left hemisphere of the brain. Each half of the observer’s brain receives information from only one half of the image. In addition, these images are distorted, because the region around the yellow spot (the fovea – where we see best; the Latin word for a hollow) forms an image which is ten times as large as that of the peripheral area. The left side of the brain only observes the left half of the image ( = the right half of what we are looking at) and this half is the right way up with the distortion removed. At the same time the right side of the brain deals with the other half of the field of view.

Note that, although the brain processes the different parts of the image in various remote locations, the two halves of the field of vision are seamlessly re-united, without any trace of a joint – amazing! This process is still far from being fully understood.

Hermann von Helmholtz (1821 – 1894), a famous physicist and physiologist of the 19th century, comparing the error count of eye imaging with that of a lens, concluded as follows in 1863: “If an optician sold me an instrument having the errors exhibited by the eye, it would be in order for me to express my dissatisfaction with the quality of his work in the strongest terms, and return his instrument forthwith.”

Helmholtz was wrong, since he only measured the performance of the lens of the eye in comparison with the light path in optical instruments. But he forgot that no technologically produced lens system can function faultlessly for the length of a human lifespan. Neither is it protected against heat and cold, dryness and humidity, shocks and dust, nor can it repair itself in the event of minor damage. Which optical instrument available at that time could adjust itself auto- matically to prevailing conditions like bright-dark contrasts, distance, and the light spectrum? And which optical system processes the data prior to transmitting it to a computer, like the eye? But remember that, as we shall see later, the brain is much more than a computer.

The Bible and the eye: All evolutionary statements about the origin of the eye notwithstanding, the Bible affirms unequivocally that the eye is uniquely the work of the Creator. Its conception and complexity defy human genius. We read in Psalm 94:9: “… he that formed the eye, shall he not see?”. If this Word is true, as I am deeply convinced it is, then any other human ideas and words about the origin of the eye are wrong from the outset.

The eye is described in the Bible as a very important organ. It cannot become satisfied (Prov 27:20), and our heart follows our eyes (Job 31:7). The German proverb “What the eye sees, the heart believes”, is derived from this fact. Being a mirror of our soul, our eyes strongly express our personality. In the Sermon on the Mount, Jesus described this truth: “The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light. But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!” (Matthew 6:22-23).

Many other biblical statements confirm that the eye expresses our innermost nature – aspects like generosity (Prov 22:9), pride and haughtiness (Ps 18:27; 131:1, Prov 6:17, and Is 10:12), idolatry (Ez 6:9), and adultery (2 Peter 2:14). Our eyes can be piercing with hatred (Job 16:9), winking with malice (Ps 35:19), or closed to the poor and needy (Prov 28:27). With our eyes we marvel at God’s works (Ps 118:23) and expect help from Him: “Unto thee lift I up mine eyes, O thou that dwellest in the heavens. Behold, as the eyes of servants look unto the hand of their masters, and as the eyes of a maiden unto the hand of her mistress; so our eyes wait upon the LORD our God, until that he have mercy upon us.” (Psalms 123:1-2). When looking up to God, we expect his help: “I will lift up mine eyes unto the hills, from whence cometh my help. My help cometh from the LORD, which made heaven and earth.” (Psalms 121:1-2).

When man fell into sin, the eyes played a significant role: “And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, …” (Genesis 3:6). The eye was the gate to sin. Samson also experienced this. His downfall was caused by his marrying a heathen woman. What decided him, was her visible attractiveness: “… she pleaseth me well.” (Judges 14:3).

Our salvation also has to do with vision. Jesus came to this world and He could be seen by human eyes. The pious Israelite Simeon had received a promise that he would not die before seeing Christ the Lord. When he held the baby Jesus in his arms, he praised God and said “For mine eyes have seen thy salvation,” (Luke 2:30).

The apostle John expressed his knowledge of Jesus as an eye-witness: “… (and we beheld his glory, the glory as of the only begotten of the Father,) ...” (John 1:14). And the salient feature of His second coming is that everybody will see Him: “Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindredes of the earth shall wail because of him.” (Rev 1:7). On that day everybody will see Him, either as Saviour or as Judge.

With enlightened eyes – such enlightenment is also a gift of God – we can know His glory and wisdom (Eph 1:17-18). And what God has prepared for us in heaven is rich and vast beyond comprehension, as described in 1 Corinthians 2:9: “Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.” Heaven is the destination of the redeemed, and when we arrive there, we will see the Lord Jesus as He is (1 John 3:2b). In this world many people suffer severe pain and misery, and the question “Why?” is cried out often. But when we arrive at our destination, everything will be made clear, because Jesus said: “And in that day ye shall ask me nothing.” (John 16:23). All suffering will end, as stated in Revelation 21:4: “And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away.”

Quotes:

Charles Darwin (1809 – 1882) in his book The Origin of the Species:

“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”

Dr. Carl Wieland, M.B., B.S., in the magazine Creation ex nihilo (Vol. 18, No. 2, 1996, p. 40):

“Eyes in different creatures are designed to meet their differing needs. Humans need good resolution and detail, whereas a fly needs speed. We see a fluorescent lamp as flickering at 10 Hz (cycles per second) but it looks stable to us at 20 Hz. A fly can detect a flicker of 200 Hz, so a normal movie would look to it like a slide show! The simple act of walking into a room and immdiately recognizing all the objects in it requires more computing power than a dozen of the world’s top supercomputers put together.”

Proverb:
“There are none so blind as those who will not see.”

French author Antoine de Saint-Exupéry (1900 – 1944):
“One can only see well through one’s heart.”

Units of length:

1 kilometre = 1 km = 1000 m
1 metre = 1 m = 100 cm
1 centimetre = 1 cm = 10 mm = 10-2 m
1 millimetre = 1 mm = one thousandth of a metre
1 mm = 1000 μm = 10-3 m
1 micrometre = 1 μm = one thousandth of a millimetre
1 μm = 1000 nm = 10-6 m
1 nanometre = 1 nm = one millionth of a millimetre
1 nm = 1000 pm = 10-9 m
1 picometre = 1 pm = one thousand millionth of a millimetre
1 pm = 0.001 nm = 10-12 m

NEXT The Ear– our highest precision sense organ