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It will perhaps be as well to detail an experiment by which the Laws of Optics may be well illustrated.
Let a square or rectangle (Fig. 1.) of wood, or any other convenient material, have its opposite sides bisected by lines , , and be correctly graduated along the top and bottom, so that the divisions, which must be equal on both lines, may be aliquot, parts tenths or hundredths, for instance, of or .
Let this rectangle be immersed vertically in water up to the line in a dark room, so that a small beam of Sun-light admitted through a shutter may just shine along its surface in a line .
There will then be observed a reflected beam along a line on the surface of the rectangle, and a refracted one down through the water, also lying just along the surface of the rectangle. Now if the distances be observed, it will be found that and are equal, and that and , which are respectively the tangents of the angles , to the radius or are so related, that if the sines of the same angles be calculated by the formula , these sines will be found to be in a certain ratio which in the case of pure water is about that of 4 to 3, or more correctly, 1,336 to 1. The experiment should be repeated several times when the Sun is at different heights, and the ratio of the sines of the angles , will be found invariably the same.
The fact of the incident, reflected, and refracted rays, , and , all lying precisely along the same plane surface, shows that those rays are all in the same plane, which is one circumstance mentioned in the Laws.
It may be necessary to observe that it is indifferent as to the directions of connected rays, which way the light is proceeding, that is, whether forwards or backwards, as any causes that act to produce a deflection from the straight course in the one case, would produce corresponding effects in the other.
