4. Professor Forbes communicated the following memorandum respecting the Polarization of Heat

Extract

“I have recently ascertained the following facts respecting heat.

“1st, That heat polarized in any plane, and then reflected from the surface of a refracting medium, changes its plane of polarization in a manner similar to what obtains in the case of light. Thus, with a thick plate of mica, which polarized homogeneous red light most completely at an incidence of about 59°, the plane of polarization of reflected polarized heat remained on the same side of the plane of reflection when the incidence was great, and was on the contrary side when the incidence was small. The limiting angle of incidence was about 57°, which therefore should be the polarizing angle of dark heat for mica. This mode of observing the polarizing angle offers some advantages above more direct methods.