446 Prof. Tyndall on Calorescence. 



concave mirror, 3 feet in diameter, was mounted on the roof of 

 the Royal School of Mines in Jermyn Street. The focus was 

 formed in a darkened chamber, in which the platinized platinum- 

 foil was exposed. Cutting off the visible rays by the solution of 

 iodine, feeble but distinct incandescence was there produced by 

 the invisible rays. 



A blackened tin tube (A B, fig. 10, Plate IV.) with square cross 

 section and open at one end, was furnished at the other with a 

 plane mirror \x y) forming an angle of 45° with the axis of the 

 tube. A lateral aperture (xo), about 2 inches square, was cut 

 out in front of the mirror. Over this aperture was placed a leaf 

 of platinized platinum. Turning the leaf towards the concave 

 mirror, the concentrated sunbeams were permitted to fall upon 

 it. In the glare of daylight it was quite impossible to see 

 whether the platinum was incandescent or not ; but placing the 

 eye at B, the glow of the platinum could be seen by reflexion 

 from the plane mirror. Incandescence was thus obtained at the 

 focus of the large mirror, X, Y, after the removal of the visible 

 rays by the iodine solution, m n. 



To obtain a clearer sky, I had this mirror transferred to the 

 garden of my friend Mr. Lubbock, near Chislehurst. The 

 effects obtained with the total solar radiation were extraordinary. 

 Large spaces of the platinum-leaf, and even thick foil, when 

 exposed at the focus, disappeared as if vaporized*. The handle 

 of a pitchfork, similarly exposed, was soon burnt quite across. 

 Paper placed at the focus burst into flame with almost explosive 

 suddenness. The high ratio which the visible radiation of the 

 sun bears to the invisible was strikingly manifested in these 

 experiments. With a total radiation vastly inferior, the invisible 

 rays of the electric light, or of the lime-light, raise platinum to 

 whiteness, while, when the visible constituents of the concen- 

 trated sunbeam were intercepted, the most that could be obtained 

 from the dark rays of the sun was a bright-red heat. The heat 

 of the luminous rays is so great as to render it exceedingly diffi- 

 cult to experiment with the solution of iodine. It boiled up in- 

 cessantly, exposure for two or three seconds being sufficient to 

 raise it to ebullition. This high ratio of the luminous to the 

 non-luminous radiation, is doubtless to be ascribed in part to the 

 absorption of a large portion of the latter by the aqueous vapour 

 of the air. From it, however, may also be inferred the enor- 

 mous temperature of the sun. 



Converging the sun's rays with a hollow lens filled with the 



* Concentrating the solar rays with a mirror 9 inches in diameter and of 

 6 inches focal length upon a leaf of platinized platinum, the metal was in- 

 stantly pierced. Causing the focus to pass along the leaf, it was cut by the 

 sunbeam as if a sharp instrument had been drawn along it. 



