To George Gabriel Stokes   3rd. Jan. 1862

Royal Institution | 3^rd. Jan. 1862.

Dear M^r. Stokes

I have received your letter¹ & read it with profit—Accept my best thanks for it. Long may you be able as the years run round to wield at the commencement of each period a pen as effectively as you do now.

Yours truly | John Tyndall

One word with regard to the influence of the rocksalt plates. Take a large surface—uniformly heated. Present your thermo-electric pile to it furnished with a conical reflector blackened within—The production of the cone intersects the surface in a circle, & this circle is the available radiating space. All parts outside of it send rays against the internal surface of the cone & such rays are there absorbed. Retreat from the surface—say to ten times the first distance—the available radiating surface increases as the square of the distance but the deflection of the galvanometer remains perfectly constant. This is a proof that the heat diminishes in intensity exactly as the square of the distance increases. When the air is very dry you may push this to any reasonable laboratory distance. Now the law could not thus clearly come out if the air had exerted a sensible absorption.

RI MS JT/T/1399