The eye is the organ that allows us to see. Light enters through the cornea and is projected onto the retina, which consists of cells that are sensitive to electromagnetic radiation. From there, the light signals are converted into electrical impulses that are transmitted to the brain along the optic nerve. Once the brain has received the pulses, it transforms the information into an image.

The human eye is spherical and its internal parts are transparent so that the maximum amount of light can reach the retina.

The cornea surrounds and protects the eye. It is made of transparent gel-like material that helps converge light rays onto the retina. The role of the iris is to control the quantity of light that passes through the cornea, keeping it at a fairly constant level. Too much light can damage the retina, whereas not enough will prevent a person from seeing properly.

Basic anatomy of the eye.The iris is nothing more than a ring of coloured muscles, and it is these muscles that give us our eye colour. Its centre, the pupil, allows just the right amount of light to pass through so that we can see. This is why the pupil contracts when there is too m Read More
The eye is the organ that allows us to see. Light enters through the cornea and is projected onto the retina, which consists of cells that are sensitive to electromagnetic radiation. From there, the light signals are converted into electrical impulses that are transmitted to the brain along the optic nerve. Once the brain has received the pulses, it transforms the information into an image.

The human eye is spherical and its internal parts are transparent so that the maximum amount of light can reach the retina.

The cornea surrounds and protects the eye. It is made of transparent gel-like material that helps converge light rays onto the retina. The role of the iris is to control the quantity of light that passes through the cornea, keeping it at a fairly constant level. Too much light can damage the retina, whereas not enough will prevent a person from seeing properly.

Basic anatomy of the eye.The iris is nothing more than a ring of coloured muscles, and it is these muscles that give us our eye colour. Its centre, the pupil, allows just the right amount of light to pass through so that we can see. This is why the pupil contracts when there is too much light (like when the Sun blinds us) to prevent excess light rays from reaching the retina, and contracts when there is not enough light (like when we walk into a darkened room) to allow in the most light possible.

Immediately behind the iris is the lens. It is a true lens in that it converges light rays passing through the eye onto the retina. The lens of the eye is also flexible and can deform itself to improve the convergence of light.

The retina consists of two types of cells: cones and rods. Cone cells have a very important role to play because they allow us to see in colour. On the other hand, they are not very sensitive to light rays and need a lot of light to work.

The rods are very sensitive to light and enable us to see when it is dark. They are not, however, capable of distinguishing different colours and details, and this causes the poor colour contrast between objects that we observe when the lighting is dim.

In all, hundreds of millions of cells must work together in our eyes and brain in order for us to see. It is thanks to their highly coordinated efforts that you are able to read these words right now!

© 2006 An original idea and a realization of the ASTROLab of Mont-Mégantic National Park

Colour photo of a human eye

The human eye.

ASTROLab of Mont-Mégantic National Park

© 2006 An original idea and a realization of the ASTROLab of Mont-Mégantic National Park


A diagram depicting the location of different parts of the eye

The Parts of the Eye.

ASTROLab of Mont-Mégantic National Park

© ASTROLab/Mont-Mégantic National Park


Many different astronomical instruments have been used throughout the ages. Two stand out as having played particularly important historical roles: the transit and the sextant. Such diverse fields as stellar cartography (mapping), navigation and the determination of time have all progressed significantly thanks to these instruments.

The transit is used to measure the precise moment that a celestial object passes through its highest point in the sky (also know as the meridian). Used since ancient Egyptian time, the modern form of this instrument consists of a telescope that only moves vertically. A dial on one side, graduated from 0 to 90 degrees, is used to measure the inclination of the telescope.

When the celestial object in question passes through the meridian, the observer notes its angle above the horizon at the precise moment of its passing. These two measurements provide the coordinates (the position) of the object in the celestial sphere. A map of the sky can be thus be assembled bit by bit and used to create tables that give the positions of stars, planets, the Moon and so on for the purposes of navigation or determining the exact time.

Jo Read More
Many different astronomical instruments have been used throughout the ages. Two stand out as having played particularly important historical roles: the transit and the sextant. Such diverse fields as stellar cartography (mapping), navigation and the determination of time have all progressed significantly thanks to these instruments.

The transit is used to measure the precise moment that a celestial object passes through its highest point in the sky (also know as the meridian). Used since ancient Egyptian time, the modern form of this instrument consists of a telescope that only moves vertically. A dial on one side, graduated from 0 to 90 degrees, is used to measure the inclination of the telescope.

When the celestial object in question passes through the meridian, the observer notes its angle above the horizon at the precise moment of its passing. These two measurements provide the coordinates (the position) of the object in the celestial sphere. A map of the sky can be thus be assembled bit by bit and used to create tables that give the positions of stars, planets, the Moon and so on for the purposes of navigation or determining the exact time.

John HadleyThe sextant is an instrument that can be used to determine the angle between a celestial object and the horizon. Used in conjunction with the transit tables, it is a simple matter to calculate the latitude of one’s position.

The predecessors of the sextant included the astrolab and the octant, both of which operated on the same principle. The modern version of the sextant was independently invented in the 1730’s by John Hadley in England and Thomas Godfrey in the United States.

© 2006 An original idea and a realization of the ASTROLab of Mont-Mégantic National Park

A colour photo of a transit

A transit.

Thomas B. Greenslade, Jr.

© Thomas B. Greenslade, Jr.


A colour photo of a sextant

A sextant.

ASTROLab of Mont-Mégantic National Park

© Public Domain


Black and white drawing of John Hadley

John Hadley.

ASTROLab of Mont-Mégantic National Park

© Public Domain


Learning Objectives

The learner will:
  • identify recent contributions, including Canada’s, to the development of space exploration technologies;
  • describe in detail the function of Canadian technologies involved in exploration of space;
  • draw a solar system with all its components;
  • establish the link between atoms and light using different instruments.

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