HEAT 393 



placed in the rays marked out by their luminous effects. If 

 non-luminous rays are to be demonstrated, the positions are 

 first marked from the luminous rays ; after which a cell of 

 iodine dissolved in carbon disulphide or tetra- chloride is 

 interposed, and the thermo-pile introduced into the marked 

 positions, the galvanometer instantly responding. When 

 the polarised ^beam is cut off by the analyser, and the latter 

 is at zero, the introduction of a plate of crystal between 

 the crossed Nicols, at once causes a vigorous movement. In 

 proving electro-magnetic rotation of the dark beam, Prof. 

 Tyndall employed, for the sake of the greater increase of 

 transmission in that position, the two Nicols (or other 

 apparatus) with their polarising planes enclosing an angle 

 of 45 instead of 90, equilibrating the thermo-pile and bring- 

 ing it to zero for the amount of heat thus transmitted, by 

 placing some other source of heat at the requisite distance 

 from the other face of the pile. Then upon switching on 

 the current, a movement of the galvanometer at once showed 

 the greater amount of heat transmitted by the rotated 

 beam. 



Using a diffraction grating and slit (as in 191) with a 

 red glass, and also with the iodine-cell, bands of action and 

 extinction as in the case of light, are readily shown by the 

 thermo-pile. It is just barely possible, with a fine slit on the 

 thermo-pile, to show heat fringes with a Fresnel prism ; but 

 so very difficult and uncertain is a clear result, that I 

 abandoned the experiment with regret. Probably others 

 with more skill in this sort of manipulation, and more 

 opportunities with the arc light than have been possible to 

 me, may have greater success. For such experiments, the 

 radio -micrometer recently perfected by Mr. C. Vernon Boys, 

 on the principle of d'Arsonval's thermo-galvanometer, 1 

 gives both more delicacy in recording small variations of tem- 



1 The fundamental principle is the suspension of a thermo-electric closed 

 circuit by a torsion-fibre in a magnetic field. 



