By Habre Nadim, Marianna DeMartini and Razan Hanna École La Dauversière, Montreal, June 2000 Content validation and linguistic revision : St é phane Lamarche  Science animée, 2000 Content validation and linguistic revision : St é phane Lamarche  Science animée, 2000 click here to begin Habre, Nadim, of Martini, Marianna and Hanna, Razan Translated from French by Nigel Ward

Structure of the eye Visual perception Circuit of the eye to the brain Vision defects Perception by the eye Seeing Colours

The eyeball possesses three membranes. -the sclera: the most external membrane, white, fibrous and very resistant, protects and gives shape to the eye. The front part of the sclera has a hole in which the transparent, dome-shaped cornea is located. -the choroid: intermediate membrane, pigmented, richly vascularised (many blood vessels). It is this membrane which nourishes the eye. The ciliary muscle and the iris are located in a hole in the front of the choroid. -the retina: the most internal membrane, it covers the back third of the eye. It supports the light-sensitive nerve cells. The light rays converge on the retina where the images are formed. click here

Return to the table of contents click on the structures Blind Spot

A white, fibrous membrane which surrounds the eyeball. The front part of the sclera has a hole in which the transparent, dome-shaped cornea is located. click to return to the eye.

The eye’s first lens, deprived of blood vessels in order to ensure its transparency to light. the yellow mark represents the part concerned. click to return to the eye.

A liquid that fills the anterior (front) chamber of the eye, between the cornea and the iris. click to return to the eye.

A thin (less than 1 mm) richly vascularised (many blood vessels) layer or membrane between the sclera and the retina. It is this membrane which nourishes the eye. the yellow mark represents the part concerned. click to return to the eye.

A ring of muscle tissue surrounding the crystalline lens. When the ciliary muscle contracts it squeezes the lens, making it rounder and allowing the eye to focus on objects that are close. click to return to the eye.

A ring of smooth, coloured muscle whose constriction and dilation controls the size of the pupil which is its central opening. the yellow mark represents the part concerned. click to return to the eye.

The central opening in the iris. The pupil widens in dim light to let more light through. the yellow mark represents the part concerned. click to return to the eye.

The eye’s internal lens. By adjusting its curvature, accommodation (focusing) is possible. The eye also possesses an external lens, the cornea. The yellow mark represents the part concerned. click to return to the eye.

Membrane sensitive to light covering the inner surface of the back of the eye, which converts the optical images into nerve signals which the optic nerve carries to the brain. the mark yellow represents the part concerned. click to return to the eye.

A yellow patch, 3 to 4 mm in diameter, situated at the centre of the back part of the retina. At the centre of the macula is the fovea. It is on the fovea that the sharpest images are formed. The majority of the cones are concentrated at this spot. The cones are the nerve cells responsible for colour vision and for the sharpness of images. The rods are the neurons which allow you to see when the light is dim but only in shades of gray. click to return to the eye.

The centre of the macula, characterised by a high concentration of cones. The cones are the cells responsible for the vision of details and colours. the yellow mark represents the part concerned. click to return to the eye.

Nerve of vision (one for each eye). Each one has about 1 million fibres carrying visual information from the retina to the brain. the yellow mark represents the part concerned. click to return to the eye.

A zone on the retina where the nerve fibres from the light-sensitive cells (rods and cones) join together to form the optic nerve. This zone has no light-sensitive cells so it is called the ‘blind spot’. click to return to the eye.

A viscous gel occupying the principal cavity of the eye, between the crystalline lens and the retina. ‘Vitreous’ means non-crystalline. The yellow mark represents the part concerned. click to return to the eye.

Perception of a distant object: When one looks at a distant object, the ciliary muscles relax causing the lens to become flatter and thinner. Light rays passing through the lens are refracted (bent) only slightly. Image

Click here to see how the eye focuses on a close object The ciliary muscles are relaxed and the crystalline lens is relatively flat. Looking at a distant object

Perception of a close object: To enable us to see objects that are close, the ciliary muscles contract, squeezing the crystalline lens and making it more rounded. The closest point that the object can be without appearing blurred corresponds to the maximum curvature of the lens. Image

Return to the table of contents Looking at a close object The ciliary muscles contract, squeezing the lens and making it more rounded.

Short sight: Short sight is due to the eyeball being too long. The image forms in front of the retina instead of on it. Solutions: Concave glasses or contact lenses can be used to correct the focus. Image of the short sight

to long sight to correct short sight: light rays cornea crystalline lens concave lens to correct short sight

Long sight: Long sight is due to the eyeball being too short. The image forms behind the retina instead of on it. Solutions: Convex glasses or contact lenses make it possible to correct the focus. Image of long sight

Use a convex lens to correct long sight. Correction of the anomalies of the eye

The ‘purest’ colours are the colours you see in a rainbow (the ones you see on the back of this slide). We often say there are seven colours in the rainbow: red, orange, yellow, green, blue indigo and violet (“Richard of York gave battle in vain!”) but in fact there are many more shades than that. Also, many other colours exist that do not appear in a rainbow (gray, brown etc.). Your eyes can distinguish more than a million different colours but actually contain only three types of colour- sensitive cells. How can that be?

Recall that CONE CELLS are the cells on the retina that are responsible for sharp colour vision in bright light (as opposed to the ROD cells which give less sharp greyscale vision in dim light). There are in fact three kinds of cone cells: one kind detects mainly short wavelength light (the blue end of the spectrum), one detects intermediate wavelengths (the green part of the spectrum) and one detects longer wavelengths (the red end of the spectrum). We can perceive more than a million different colours because each colour stimulates the three types of cones in a different way. Pure yellow light, for example, stimulates the red and green detectors. This means that by combining red, green and blue light together in different combinations we can ‘trick’ our eye into seeing any colour that it is capable of seeing. For example, if the eye receives a combination of red and green light then the eye will see yellow – it is impossible for the eye to tell the difference between this ‘compound yellow’ and the pure yellow found in a rainbow, for example, since they both stimulate the red and green receptors in the same way.

By adding pairs of primary colours together we obtain the SECONDARY COLOURS yellow, cyan and magenta. Pairs of colours on opposite sides of the picture, such as yellow and blue, are known as COMPLEMENTARY COLOURS. If you are looking at this on a computer screen then look very closely and you will see that the screen consists of red, green and blue dots only – if you look at the yellow part of this picture you will see that the red and green Since humans eyes have cells that respond to red, green and blue light, scientists call these colours the PRIMARY COLOURS (in art class you may have been given a different set of primary colours but that is because art class uses paint pigments which absorb (subtract) colours whereas scientists are more interested in adding coloured lights together). The image below shows what you would see if you projected circular red, blue and green light beams onto a white screen such that they overlap. dots are glowing in that area – this combination gives yellow. Any colour you have ever seen can be reproduced by mixing red, blue and green in the right proportions.