388 Prof. Tyndall on Calorescence. 



pile devised by Melloni for the examination of this and kindred 

 questions. Through the kindness of my friend Mr. Gassiot, a 

 very beautiful instrument of this kind, constructed by Ruhm- 

 korff, has remained in my possession for several years, and been 

 frequently employed in my researches. It consists of a double 

 metallic screen, with a rectangular aperture in the centre, a sin- 

 gle row of thermo-electric elements 1*2 inch in length being 

 fixed to the screen behind the aperture. Connected with the 

 latter are two moveable side pieces, which can be caused to ap- 

 proach or recede so as to vary the width of the exposed face of 

 the pile from zero to ^th of an inch. The instrument is 

 mounted on a slider, which, by turning a handle, is moved along 

 a slot on a massive metal stand. A spectrum of a width equal 

 to the length of the thermo-electric pile being cast at the proper 

 elevation on the screen, by turning the handle of the slider the 

 vertical face of the pile can be caused to traverse the colours, 

 and also the spaces right and left of them. 



To produce a steady spectrum of the electric light, I employed 

 the regulator devised by M. Foucault and constructed by ])u- 

 boscq, the constancy of which is admirable. A complete rock- 

 salt train was constructed for me by Mr. Becker, and arranged 

 in the following manner : — In the front orifice of the camera 

 which surrounds the electric lamp was placed a lens of trans- 

 parent rock-salt, intended to reduce to parallelism the divergent 

 rays proceeding from the carbon-points. The parallel beam was 

 permitted to pass through a narrow slit, in front of which was 

 placed another rock-salt lens, the position of this lens being so 

 arranged that a sharply-defined image* of the slit was obtained 

 at a distance beyond it equal to that at which the spectrum was 

 to be formed. Immediately behind this lens was placed a pure 

 rock-salt prism (sometimes two of them). The beam was thus 

 decomposed, a brilliant horizontal spectrum being cast upon the 

 screen which bore the thermo-electric pile. By turning the 

 handle already referred to, the face of the pile could be caused 

 to traverse the spectrum, an extremely narrow band of light or 

 radiant heat falling upon it at each point of its march f. A 

 sensitive galvanometer was connected with the pile, and from its 

 deiicction the heating-power of every part of the spectrum, visible 

 and invisible, was determined. 



Two modes of moving the instrument were practised. In the 

 first the face of the pile was brought up to the violet end of the 

 spectrum, where the heat was insensible, and then moved through 

 the colours to the red, then past the red up to the position of 

 maximum heat, and afterwards beyond this position until the 

 heat of the invisible spectrum gradually faded away. The fol- 



* The width of the image was about 0*1 of an inch, 

 t The width of the linear pile was 003 of an inch. 



