Page:Proceedings of the Royal Society of London Vol 4.djvu/333

Rh at 83½°, he concludes that the compensation which takes place i& produced neither by an equality of oppositely polarized rays, nor by a proportional admixture of common light, but by equal and opposite physical states of the whole pencil, whether reflected or refracted.

The remarkable phenomena produced at an angle of incidence on glass of 82° 44' (at which angle cos (i + i') = cos^ (i -j- i')), led the author to the construction of what he terms the compensating rhomb, consisting of a well annealed rhomb of glass, or any other uncry- stallized substance, having the angles of its base 130° 25' and 46° 35' respectively, when the index of refraction is r525. When a ray of light is incident upon the first surface at an angle of 82° 44', ex- actly one-half of it is reflected ; and the other half, after refraction, is reflected at the second surface, and emerges perpendicularly to the adjacent surface, without sufi'ering refraction; each portion having, in the first instance, the same quantity of polarized light. The second portion is found, on examination, to be in the state of common light, although the ray at the second incidence consisted of more than one -half of polarized light. Hence if the pencil, pre- viously to reflexion at the second surface, consist of 145 rays of polarized light, and 188 of common light, the effect of a single re- flexion must be to depolarize polarized light, and to produce no change whatever upon common light, a property of a reflecting surface never yet recognized, and incompatible with all our present knowledge on the subject of the polarization of light.

The author then describes an instrument which he has invented for the purpose of accurately measuring the degrees of polarization, and which he therefore terms a Polarimeter. It consists of two parts ; one of which is intended to produce a ray of compensation having a physical character susceptible of numerical expression, and the other to produce polarized bands, or rectilineal isochromatic lines, the extinction of which indicates that the compensation is effected. The construction and mode of operation of this instrument are, by the aid of figures, described and explained.

The following is the general law established by these researches ; namely, that the compensations of polarized light are produced by equal and opposite rotations of the planes of polarization. Thus, when a ray of common light is incident, at any angle, upon the polished surface of a transparent body, the whole of the reflected pencil suffers a physical change, bringing it more or less into a state of complete polarization, in virtue of which change its planes of polari- zation are more or less turned into the plane of reflexion ; while the whole of the refracted pencil has suflfered a similar, but opposite change, in virtue of which its planes of polarization are turned more or less into a plane perpendicular to the plane of reflexion.

The author then enters into a theoretical investigation of the sub- ject, and concludes by pointing out a few of the numerous applica- tions of his theory.

2. Continuation of the paper of which the reading commenced