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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.