light: 1. We have a clear glass bottle, with a sheet of white paper, in which a perfectly round hole has been cut, pasted on its side; 2. Horizontal and perpendicular lines are drawn with ink upon the glass at right angles across each other, and within this circle, dividing it into four equal parts; 3. Water is poured into the bottle until its level is that of the horizontal line; 4. A postal-card containing a slit is placed as at D in the figure; 5. The mirror, B, reflects the beam into the bottle so that it may touch the water where the two lines cross. The light is seen to bend as soon as it enters the water. The index of refraction for all liquids may be determined by measuring the distance from where the beam enters to the perpendicular, then the distance from the perpendicular to where it vanishes, and dividing these into each other. The constancy of the quotient for each particular liquid can also be shown by having the beam strike the water at various angles, making these measurements and dividing. No matter how the distances vary from the perpendicular, when divided into each other they give the same result for the same liquid.
Fig. 6.—Total Reflection.
With a mirror on the table and our bottle arranged as in Fig. 6, total reflection will occur, that is, all the light of the beam will be thrown downward from the surface of the water.
