398 



of sharpness, and singular beauty and delicacy of tint. From his 

 further researches on this subject he deduces the following conclu- 

 sions : first, that it is the heat of the rays, not their light, which ope- 

 rates the change ; secondly, that this heat possesses a peculiar che- 

 mical quality, which is not possessed by the purely calorific rays 

 outside of the visible spectrum, though far more intense ; and thirdly, 

 that the heat radiated from obscurely hot iron abounds especially in 

 rays analogous to th )se of the region of the spectrum above de- 

 scribed. 



The author then describes the photographic properties he has 

 discovered to belong to mercury, a metal which he finds to possess, 

 in an eminent degree, direct photographic susceptibility. 



2. "Observations de la variation de la declinaison et intensite hori- 

 zontale magnetiques observees a Milan pendant vingt-quatre heures 

 consecutives, le 22 et 23 Juin, le 20 et 21 Juillet, le26 et 27 d'Aout, 

 le 21 et 22 Septembre, et le 19 et 20 Octobre, 1842," rapportees 

 par Robert Strambrecchi, premier eleve adjoint. 



A letter was also read from Sir John F. V/. Herschel on the sub- 

 ject of Photography, addressed to S. Hunter Christie, Esq., Sec. R.S. 



November 24, 1842. 

 FRANCIS BAILY, Esq., Vice-President, in the Chair, 

 The following papers were read, viz : — 



1. " On certain improvements on Photographic Processes described 

 in a former communication." By Sir John Frederick William Her- 

 schel, Bart, K.H., F.R.S., &c., in a letter to Samuel Hunter Christie, 

 Esq., Sec. R.S. Communicated by Mr. Christie. 



The present memoir, which is a sequel to the last by the same 

 author, is accompanied by a series of photographic impressions illus- 

 trative of the chrysotype, cyanotype, and other processes formerly 

 described by him. Some improvements which he has introduced 

 into these processes are given, together with a few remarks on some 

 other points treated of in the former paper, in relation to the influ- 

 ence of thermic rays as distinct from calorific rays ; the former being 

 rays, which- in the spectrum accompany the red and orange rays, 

 which are also copiously emitted by heated bodies short of redness, 

 and which are distinguished from those of light by being invisible. 

 The author thinks they may be regarded as bearing the same rela- 

 tion to the calorific spectrum which the photographic rays do to the 

 luminous one, and would proiiose to designate them by the term 

 parathermic rays. He conceives that these may be the rays which 

 are active in producing those singular molecular afiections determi- 

 ning the precipitation of vapours in the experiments of Messrs. 

 Draper, Moser, and Hunt, and which will probably lead to import- 

 ant discoveries as to the intimate nature of those forces, resident on 



