In a paper on the Polarisation of Light by Refraction, published in the “Philosophical Transactions” for 1814, I have shown that when a pencil of light is incident on a number of uncrystallised plates, inclined at the same or different angles to the incident ray, all their surfaces being perpendicular to the plane of the first incidence, the transmitted pencil will be wholly polarised, when the sum of the tangents of the angle of incidence upon each plate is equal to a constant quantity, depending upon the refractive power of the plates and the intensity of the incident pencil.