In the same manner, if from the point B, upon the right line I K, there be let fall the perpendicular B E, that line will be the sine of the angle of refraction.
The sines of the angles are the measures of the refractions, and this measure is constant; that is, whatever is the sine of the angle of incidence, it will be in a constant proportion to the sine of the angle of refraction, when the mediums continue the same. A general idea of refraction may be formed from the following experiments.
Experiment 1. Let A B C D, Fig. 3. [Plate I.] represent a vessel so placed, with respect to the candle E, that the shadow of the side A C may fall at D. Suppose the vessel to be now filled with water, and the shadow will withdraw to d; the ray of light, instead of proceeding to D, being refracted or bent to d. And there is no doubt but that an eye, placed at d, would see the candle at e, in the direction of the refracted ray d A. This is also confirmed by the following pleasing experiment.
2. Lay a shilling, or any piece of money, at the bottom of a bason; then withdraw from the bason, till you lose sight of the shilling; fill the bason nearly with water, and the shilling will be seen very plainly, though you are at the same distance from it.
3. Place a stick over a bason which is filled with water; then reflect the sun’s rays, so that they may fall perpendicularly on the surface of the water; the shadow of the stick will fall on the same place, whether the vessel be empty or full.
What has been said of water, may be applied to any transparent medium, only the power of refraction is greater in some than in others. It is from this wonderful property, that we derive all the curious effects of glass, which make it the subject of optics. It is to this we owe the powers of the microscope and the telescope.
To produce these effects, pieces of glass are formed into given figures, which, when so formed, are called lenses. The six following figures are those which are most in use for optical purposes.
1. A PLANE GLASS, one that is flat on each side, and of an equal thickness throughout. F, Fig. 13. [Plate I.]
2. A DOUBLE CONVEX GLASS, one that is more elevated towards the middle than the edge. B, Fig. 13. [Plate I.]
3. A DOUBLE CONCAVE is hollow on both sides, or thinner in the middle than at the edges. D, Fig. 13. [Plate I.]