Download 6 Illusions

By Catherine Grenier and Érika Fitsimmons
École Joseph-François Perrault, Montreal, June 2001
Content validation and language revision: Leïla Touta
Science
animée, 2001
Translated from French by Nigel Ward
Click here to begin
Contents
Introduction
Structure of the eye
some diseases of the eye
optical illusions
Bibliography
Click a heading
The structure of the eye
The cornea, iris and sclera (the white part of the eye) are the
external parts of the eye and are thus visible.
The retina, lens, macula and optic nerve are inside.
Transparent parts of the eye
The transparent media of the eye transmit light to the
retina. The transparent media are listed here in order,
from the exterior to the interior:

the cornea
 the aqueous humour
 the crystalline lens
 the vitreous humour
Structure of the eye
The membranes are listed here in order, starting with the outside of
the eye and working inwards.
the sclera: a white resistant membrane which protects the eye
the choroid: a membrane with many blood vessels, it nourishes
the eye
the retina: the light-sensitive membrane of the eye, it contains
nerve cells that absorb light and transform it into nerve impulses.
The optic nerve transmits these nerve signals to the visual
region of the brain.
detachment of the retina
squinting
styes and chalazions
Click a heading
Detachment of the retina
the retina is, as you know, like a cinema screen and covers the
inner surface of the back of the eye. If this screen becomes detached
from its support we call this ‘detachment of the retina’.
it is a serious condition which can lead to blindness if not treated.
it occurs between the ages of 45 and 60.
it is a painless condition.
Detachment of the retina
What causes the detachment ?
short-sightedness
dangerous degenerative lesions of the retina
the detachment of the back of the vitreous humour from
the retina
aphakia (the absence of the crystalline lens)
ocular traumatisms
personal or family history of detachment of the retina
Detachment of the retina
the symptoms
perception of dark or irregular images, due to objects floating in
the vitreous humour.
impression of flying insects and of coloured lightning (this is a
warning sign but detachment has not yet occurred).
impression of looking through a red veil.
significant loss of sharpness of central vision.
treatment
surgery
immobilisation of the eye before and after the operation is important
treatment of the cause of the detachment may be attempted.
Return to diseases
Squinting
(being cross-eyed)
squinting is the deviation of one of the two eyes which no longer looks in
the same direction as the other.
it is accompanied by visual disturbances.
it is a common disorder (4% of children), which generally appears before
the age of 4.
there is a significant genetic predisposition: 65% of squinting children
come from families where squinting is present.
it is a serious condition which in 65% of cases can lead to the loss of
vision in one eye – this is called amblyopia or ‘lazy eye’.
this amblyopia must be treated very early. After 4 or 5 years, there is
minimal hope of healing.
Squinting
the causes
Sometimes no cause can be identified.
It can be a knock-on effect of ocular problems such as a cataract which is an
obstacle to the visual perception.
evolution
So as not to see double, the child uses only one eye, the one that is not deviated.
The brain is stimulated only by that one eye.
If the squinting is not treated then the unused eye becomes blind.
After the age of 6 years old, vision can no longer be restored to the blind eye.
Squinting
Treatment
First stage: the wearing of special glasses to reduce the angle of
deviation
Second stage (only performed if a visible ocular deviation
persists): surgical treatment.
Treatment of amblyopia: is a very important stage to obtain the
same level of sharpness of vision in both eyes. This last treatment
must be continued for several years to avoid a relapse of the
amblyopia.
Return to diseases
Styes and chalazions
These are diseases of the eyelids.
In our eyelids we have, in addition to skin and muscle, eyelashes and oil-producing
glands, called Meibomius glands. Meibomius glands are a specialised form of
sebaceous gland. Every hair on your body has a sebaceous gland near its base
which coats the hair with an oily substance called sebum. The Meibomius glands
don’t have hairs, instead they squeeze sebum into the tear liquid that coat your eye
to slow down its evaporation.
A stye is a boil on the eyelid. A boil is an inflammation of a hair follicle caused by a
staphylococcus bacterium.
Chalazion is an inflammation of the Meibomius gland (see photo) caused by the
blocking of the duct which drains the gland.
symptoms
The stye: an infection of the sebaceous glands at the base of the eyelashes.
While they produce no lasting damage, they can be quite painful.
A chalazion: a small hard lump under the skin, not very painful.
evolution
The stye: the pus comes out by itself and the stye dries out.
The chalazion: can disappear by itself. But it can come back or get infected.
treatment
Stye: the local application of hot, humid compresses
several times per day and application of an antibiotic
ophthalmic cream.
Chalazion: application of an anti-inflammatory
antibiotic cream for 2 weeks.
If the nodule does not regress it is then necessary to
remove it by surgery.
Return to diseases
Stare at the black points for 30 seconds
and then close your eyes
What do you see?
Musician or woman?
Are the lines straight or curved?
What colour are the circles?
If you cannot see a number in each of the pictures
below then you have some kind of colour blindness
and should have a checkup. (These tests only work if
you are looking at this page in colour, of course!)
Shifting gears: Afterimages of complementary colors create
apparent movement in our peripheral vision as our eyes
shift across the page.
The red squares are the same color in the
upper part and in the lower part of the "X“.
Three Streams: Apparent movement of the
streams is created by afterimages as our
eyes shift to examine the picture.
Warped Squares? There are no curved lines in
these figures. You can use a ruler to check it
out. The diagonal patterns created by the tiny
squares distort the perception of the pictures.
Checkerboard with shadow
The squares labeled A and B are the same shade of gray. The
illusion that B is lighter than A is caused by the relative contrast
of the surrounding dark squares and by the fact that our vision
compensates for the shadow of the cylinder.
There are no gray spots at the corners of
the squares.
WORD COLOUR TEST
In this test DO NOT READ the words, just say
aloud the COLOUR of each word.
YELLOW BLUE ORANGE
BLACK RED GREEN
PURPLE YELLOW RED
ORANGE GREEN BLACK
BLUE RED PURPLE
GREEN BLUE ORANGE
This is a type of psycholinguistic test that poses some
difficulty because the portion of the brain that handles
language has the conflicting tasks of verbalizing the
colour of the written words while ignoring the meaning of
words representing colours.
Perpetually ascending staircase
How can the man go up all the time? Can
such a staircase be built as a real object?
If you enjoy optical illusions check out
http://www.scientificpsychic.com/graphics/
Stare at the black dot, then approach the screen then move away !!
Bibliography
BRES, Stéphane, CHAMPIN Pierre-Antoine, HERAUD Jean-Mathias, HERILIER Vincent, JOLION
Jean-Michel LOUPIAS Etienne. (Page consulted 25 April 2001). Traitement d’images et vision
artificielle, [Online]: http://telesun.insa-lyon.fr
Kerignard, Philippe, Meyer, Lucie. (Page consulted 30 April 2001). Illusions d’optique, [online]:
http ://www.chez.com/kerignard/optical35.htm
Medisite. (Page consulted 25 April 2001). Maladies des yeux, [online]:
http://www.medisite.fr/pathologies
BERTORELLO, Serge. (Page consulted 15 May 2001). Techniques d’astronomie, [online]:
http://serge.bertorello.free.fr
Bibliography
Cercle d'Action pour le Dépistage des Troubles visuels. (Page consulted 26 April 2001).
Amblyopie et Strabisme, [On line]: http://perso.wanadoo.fr/strabismecadet/#somm2
Rossant, Lyonel, Jacqueline Rossant- Lumbroso. (Page consulted 22 April 2001).
le detachement rétinien, [On line]:
http://www.doctissimo.fr/html/sante/encyclopedie/sa_1572_decol_retine.htm