Prof. Tyndall on Calorescence. 439 



A piece of platinized platinum-foil was supported in an ex- 

 hausted receiver, the vessel being so placed that the focus fell 

 upon the platinum. The heat of the focus was instantly con- 

 verted into light, a clearly- denned and inverted image of the 

 points being stamped upon the metal. Fig. 5 (Plate IV.) repre- 

 sents the thermograph of the carbons. 



Blackened paper was now substituted for the platinum in the 

 exhausted receiver. Placed at the focus of invisible rays, the 

 paper was instantly pierced, a cloud of smoke was poured though 

 the opening, and fell like a cascade to the bottom of the receiver. 

 The paper seemed to burn without incandescence. Here also a 

 thermograph of the coal-points was stamped out. When black 

 paper is placed at the focus, where the thermal image is well 

 denned, it is always pierced in two points, answering to the 

 images of the two carbons. The superior heat of the positive 

 carbon is shown by the fact that its image first pierces the 

 paper ; it bums out a large space, and shows its peculiarly 

 crater-like top, while the negative carbon usually pierces a small 

 hole. 



Paper reddened by the iodide of mercury had its colour dis- 

 charged at the places on which the invisible image of the coal- 

 points fell upon it. 



Disks of paper reduced to carbon by different processes were 

 raised to brilliant incandescence, both in the air and in the 

 exhausted receiver. 



In these earlier experiments I made use of apparatus which 

 had been constructed for other purposes. The mirror, for ex- 

 ample, was detached from a Duboscq's camera, first silvered at 

 the back, but afterwards silvered in front. The cell employed 

 for the iodine solution was also that which usually accompanies 

 Duboscq's lamp, being intended by its maker for a solution of 

 alum. Its sides are of good white glass, the width from side 

 to side being 1*2 inch. 



§ 5. A point of considerable theoretic importance was involved 

 in these experiments. In his excellent researches on fluores- 

 cence, Professor Stokes had invariably found the refrangibility 

 of the incident light to be lowered. This rule was so constant 

 as almost to enforce the conviction that it was a law of nature. 

 But if the rays which in the foregoing experiments raised plati- 

 num and gold and silver to a red heat were wholly extra-red, 

 the rendering visible of the metallic films would be an instance 

 of raised refrangibility. 



And here I thought it desirable to make sure that no trace of 

 visible radiation passed through the solution, and also that the 

 invisible radiation was exclusively extra-red. 



This latter condition might seem to be unnecessary, because 



2G2 



