20-i ANNUAL OF SCIENTIFIC DISCOVERY. 



like every natural truth that can be brought to the test of experiment, 

 the verity of Sir William Hersehei's announcement was soon com- 

 pletely established. Forty years after the discovery of those invisible 

 rays by his father, Sir John Herschel made them the subject of exper- 

 iment. He made an arrangement which enabled him to estimate tlie 

 heating power of the spectrum by its drying power. Wetting by a 

 wash of alcohol paper blackened on one side, he cast his spectrum on 

 this paper, and observed the chasing away of the moisture by the heat 

 of the rays. His drying paper presented to him a thermograph of 

 the spectrum, and showed the heating power to extend far beyond the 

 red. 



By the introduction of the thermo-electric pile Melloni created a 

 new epoch in researches on radiant heat. This instrument enables us 

 to examine, with a precision unattainable with ordinary thermometers, 

 the distribution of heat in the solar spectrum. Melloni himself devot- 

 ed some time to this subject. He had made the discovery that various 

 substances, in the highest degree transparent to light, were eminently 

 opaque to those invisible heat-rays. Pure water, for example, is a 

 body of this kind. Only one substance did Melloni find to be equally 

 pervious to the visible and the invisible rays, namely, transparent 

 rock salt. And though the researches of MM. Provostaye and De- 

 sains, together with some extremely suggestive experiments executed 

 by Mr. Balfour Stuart, show conclusively that Melloni erred in sup- 

 posing rock salt to be perfectly transparent, it must be admitted that, 

 in this respect, the substance approaches very near perfection. 



Abandoning prisms of glass, which had been always employed pre- 

 viously, Melloni made use of a prism of rock salt in his experiments 

 on the solar spectrum. He was thus enabled to prove that the ultra- 

 red rays, discovered by Sir William Herschel, formed an invisible 

 spectrum, at least as long as the visible one. He also found the posi- 

 tion of maximum radiant power to lie as far on one side the red as 

 the green light of the spectrum on the other. 



Dr. Franz of Berlin subsequently examined the distribution of heat 

 in the solar spectrum, employing lor this purpose a ilint-glass prism. 

 He showed that the inaction of the ultra-red rays upon the retina did 

 not arise from the absorption of those rays in the humors of the eye ; 

 at all events he proved that a sensible, portion of the invisible rays 

 was transmitted across the eye-ball of an ox, and reached the back of 

 the eye. Professor Miiller of Freiburg afterwards examined very fully 

 the heat of the solar spectrum ; and representing, as Sir William Iler- 

 schel also had approximately done, by lines of. various lengths the ther- 

 mal intensity at various points, he drew a curve which expressed the 

 calorific action of the entire spectrum. 



At various intervals during the last ten years Prof. Tyndall has 

 occupied himself with the invisible radiation of the electric light ; and 

 to the distribution of heat in its spectrum he directed attention in a 

 recent discourse given before the lioyal Institution. The instruments 

 made use of were the electric lamp of Duboscq and the linear 

 thermo-electric pile of Melloni. The spectrum was formed by means 

 of lenses and prisms of pure rock-salt. It was equal in Avidth to the 

 length of the row of elements forming the pile, and the latter being 

 caused to pass through its various colors in succession, and also to 



