Molar

§ 82. The subject of this Lecture when originally given was “Reflection and Refraction of Light.” I have recently found it convenient to omit “Refraction” from the title because (§ 130 below), if the reflecting substance is transparent, and if we know the laws of propagation of ethereal waves through it, we can calculate the amount and quality of the refracted light for every quality of incident light, and every angle of incidence, when we know the amount and quality of the reflected light. When the reflecting substance is opaque there is no “refracted” light; but, co-periodic with the motion which constitutes the incident light, there is a vibratory motion in the ether among the matter of the reflecting body, diminishing in amplitude according to the exponential law, ∊−mD, with increasing distance D from the interface. When m−1 is equal to 10,000 wave-lengths, say half-a-centimetre, the amplitude of the disturbance at onehalf centimetre inwards from the interface would be ∊−1 of the amplitude of the entering light, and the intensity would be ∊−2, or 1/7·39, of the intensity of the entering light. The substance might or might not, as we please, be called opaque; but it would be so far from being perfectly opaque that both theoretically and experimentally we might conveniently deal with the case according to the ordinary doctrine of “reflected” and “refracted” light.