[ 386 ] 



LVIII. On Calorescence. 

 By Professor J. Tyndall, LL.D. Camb., F.R.S. %c* 



Forsitan et rosea, sol alte lampade lucens 

 Possideat multum caecis fervoribus ignera 

 Circum se, nullo qui sit fulgore notatus, 

 iEstiferum ut tantum radiorum exaugeat ictum. 



Lucret. v. 610f. 



§ 1. TN the year 1800, and in the same volume of the Philo- 

 J- sophical Transactions that contains Volta's celebrated 

 letter to Sir Joseph Banks on the Electricity of Contact J, Sir 

 William Herschel published his discovery of the invisible rays 

 of the sun. Causing thermometers to pass through the various 

 colours of the solar spectrum, he determined their heating-power, 

 and found that this power, so far from ending at the red extre- 

 mity of the spectrum, rose to a maximum at some distance 

 beyond the red. The experiment proved that, besides its lumi- 

 nous rays, the sun emitted others of low refrangibility, which 

 possessed great calorific power, but were incompetent to excite 

 vision. 



Drawing a datum-line to represent the length of the spectrum, 

 and erecting at various points of this line perpendiculars to re- 

 present the calorific intensity existing at those points, on uniting 

 the ends of the perpendiculars Sir William Herschel obtained 

 the subjoined curve (fig. 1), which shows the distribution of 



Fig. 1. 



Spectrum of Sun (Herschel) reduced, 

 heat in the solar spectrum, according to his observations. The 

 space ABD represents the invisible, and B D E the visible radi- 

 ation of the sun. With the more perfect apparatus subsequently 

 devised, Professor Muller of Freiburg examined the distribution 

 of heat in the spectrum §, and the results of his observations are 



* From the Philosophical Transactions for 1866, Part I. Communicated 

 b)' the Author. 



t I am indebted to my excellent friend Sir Edmund Head for this ex- 

 tract, which reads like divination. 



X Vol. lxx. § Philosophical Magazine, S. 4. vol. xvii. p. 242. 



