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ing the surface, but part enters the glass or water, not however without deviating from its former direction. It is in fact bent or refracted, so as to be more nearly perpendicular to the surface, so that the angle between an incident ray and a perpendicular to the surface, called as before, the angle of Incidence, is greater than that between the refracted ray and the perpendicular; this latter angle is technically termed the angle of Refraction.
Observations similar to those alluded to in the former case lead us to the conclusion that the angles of incidence and refraction are always in the same plane, and that though they do not bear an invariable ratio to one another, their sines do, provided the observations are confined to one medium, or transparent substance.
5.We have then these three laws upon which to found our theory.
- The rays of light are straight lines.
- The angles of incidence and reflexion are in the same plane and equal.
- The angles of incidence and refraction are in the same plane, and their sines bear an invariable ratio to one another for the same medium.
Note. Sir I. Newton attempted to explain the Theory of Optics on the hypothesis that light is a material substance emitted from luminous bodies, and that the minute particles of this matter are attracted by any substance on which they fall so as to be diverted from their natural straight course. He succeeded in demonstrating the laws above-mentioned upon that hypothesis, but not so as to set the question at rest. Other philosophers, probably with more truth, have supposed light to consist in undulations, or pulses propagated in a very rare and elastic medium which is supposed to pervade all space, and perhaps to have an intimate connexion with the electro-magnetic fluid.
The action of light is by no means instantaneous. It has been discovered by means of observations on eclipses of Jupiter's satellites, that light takes eight minutes, thirteen seconds of time, to come from the Sun to the Earth.
