Prof. Tyndall on Calorescence. 393 



is most distant from the red. On both sides, however, we have 

 a continuous falling off. I have had numerous experiments made 

 to ascertain whether there is any interruption of continuity in 

 the calorific spectrum; but all the measurements hitherto executed 

 with artificial sources reveal a gradual and continuous augmenta- 

 tion of heat from the point where it first becomes sensible up 

 to the maximum. Sir John Herschel has shown that this is not 

 the case with the radiation from the sun when analyzed by a 

 flint-glass prism. Permitting the solar spectrum to fall upon a 

 sheet of blackened paper, over which had been spread a wash of 

 alcohol, this eminent philosopher determined by its drying-power 

 the heating-power of the spectrum. He found that the wet sur- 

 face dried in a series of spots representing thermal maxima sepa- 

 rated from each other by spaces of comparatively feeble calorific 

 intensity. No such maxima and minima were observed in the 

 spectrum of the electric light, nor in the spectrum of a platinum 

 wire raised to a white heat by a voltaic current. Prisms and 

 lenses of rock-salt, of crown glass, and of flint glass were employed 

 in these cases. In subsequent experiments the beam intended 

 for analysis was caused to pass through layers of water and other 

 liquids of various thicknesses. Gases and vapours of various 

 kinds were also introduced into the path of the beam. In all 

 cases there was a general lowering of the calorific power, but the 

 descent of the curve on both sides of the maximum was un- 

 broken*. 



§ 3. The rays from an obscure source cannot compete in point 

 of intensity with the obscure rays of a luminous source. No 

 body heated under incandescence could emit rays of an intensity 

 comparable to those of the maximum region of the electric spec- 

 trum. If, therefore, we wish to produce intense calorific effects 

 by invisible rays, we must choose those emitted by an intensely 

 luminous source. The question then arises, how are the invisible 

 calorific rays to be isolated from the visible ones. The interpo- 

 sition of an opake screen suffices to cut off the visible spectrum 

 of the electric light, and leaves us the invisible calorific rays to 

 operate upon at our pleasure. Sir William Herschel experi- 

 mented thus when he sought, by concentrating them, to render 

 the invisible rays of the sun visible. But to form a spectrum in 

 which the invisible rays shall be completely separated from the 

 visible ones, a narrow slit or a small aperture is necessary; and 

 this circumstance renders the amount of heat separable by pris- 

 matic analysis very limited. If we wish to ascertain what the 

 intensely concentrated invisible rays can accomplish, we must 

 devise some other mode of detaching them from their visible 



* At a future day I hope to subject this question to a more severe ex- 

 amination. 



