Prof. Tyndall on the Polarization of Heat. 281 



scure rays of luminous sources were in part transmitted by black 

 glass. Intercepting by a plate of this glass the light emitted by his 

 oil lamp and operating upon the transmitted heat, he obtained 

 effects exceeding in magnitude any that could be obtained by 

 means of the radiation from obscure sources. The possession 

 of a more perfect ray-filter and a more powerful source of heat 

 enables us now to obtain, on a greatly augmented scale, the 

 effects obtained by Forbes and Melloni. 



Two large NicoPs prisms, such as those employed in my experi- 

 ments on the polarization of light by nebulous matter, were placed 

 in front of an electric lamp, and so supported that either of them 

 could be turned round its horizontal axis. The beam from the 

 lamp, rendered slightly convergent by the camera-lens, was sent 

 through both prisms. But between them was placed a cell con- 

 taining iodine dissolved in bisulphide of carbon in quantity suf- 

 ficient to quench the strongest solar light. Behind the prisms 

 was placed a thermo-electric pile, furnished with two conical 

 reflectors. The hinder face of the pile received heat from a 

 platinum spiral through which passed an electric current regu- 

 lated by a rheostat. 



The apparatus was so arranged that, when the principal sec- 

 tions of the Nicols were crossed, the needle of the galvanomete 

 connected with the pile showed a deflection of 90° in favour of 

 the posterior source of heat. One of the prisms was then turned 

 so as to render the principal sections parallel. The needle imme- 

 diately descended to zero, and passed on to 90° at the other 

 side of it. Reversing, or continuing the motion, so as to render 

 the principal sections again perpendicular to each other, the ca- 

 lorific sheaf was intercepted, the needle descended to zero and 

 went up to its first position. 



So copious indeed is the flow of polarized heat that a prompt 

 rotation of the Nicol would cause the needle to spin several 

 times round over its graduated dial. 



These experiments were made with the delicate galvanometer 

 employed in my researches upon radiant heat. But the action 

 is so strong as to cause a coarse lecture-room galvanometer, 

 with needles 6 inches long and paper indexes a square inch 

 each in area, to move through an arc of nearly 180°. 



Reflection, refraction, dispersion, polarization (plane and cir- 

 cular), double refraction, the formation of invisible images both 

 by mirrors u and lenses, may all be strikingly illustrated by the 

 employment of the iodine filter and the electric light. 



Take, for example, the following experiments : — The Nicols 

 being crossed, the needle of the galvanometer pointed to 78° 

 in favour of the heated platinum spiral behind the pile. A 

 plate of mica was then placed across the dark beam with its prin- 



