20 PROFESSOE TYNDALL ON CALOEESCENCE. 



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, simi- 

 larly 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 concentrated 

 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 ex- 

 ceedingly difficult to experiment with the solution of iodine. It boiled up incessantly, 

 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 enormous temperature of the sun. 



Converging the sun's rays with a hollow lens filled with the solution of iodine, 

 incandescence was obtained at the invisible focus of the lens on the roof of the Eoyal 

 Institution. 



Knowing the permeability of good glass to the solar rays, I requested Mr. Mayall to 

 permit me to make a few experiments with his fine photographic lens at Brighton. 

 Though exceedingly busy at the time, he in the kindest manner abandoned to my 

 assistant, Mr. Barrett, the use of his apparatus for the three best hours of a bright 

 summer's day. A red heat was obtained at the focus of the lens after the complete with- 

 di-awal of the luminous portion of the radiation. 



Black paper has been very frequently employed in tlie foregoing experiments, the 

 action of the invisible rays upon it being most energetic. This suggests that the absorp- 

 tion of those rays is not independent of colour. A red powder is red because of the 

 entrance and absorption of the luminous rays of higher refrangibility than the red, 

 and the ejection of the unabsorbed red light by reflexion at the limiting surfaces of the 

 particles of the red body. This feebleness of absorption of the red rays extends to the 

 rays of greater length beyond the red ; and the consequence is that I'ed paper when 

 exposed at the focus of invisible rays is scarcely charred, when black paper bursts in a 

 moment into flame. The following Table exhibits the condition of paper of various 

 kinds when exposed at the dark focus of an electric light of moderate intensity. 



* Concentrating the solar rays with a mirror 9 inches in diameter and of G inches focal length upon a leaf 

 of platinized platinum, the metal was instantly pierced. Causing the focus to pass along the leaf, it was cut by 

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



