422 Mr. Hunt on the Spectral linages qfM. Moser, 



rays of light) pass very readily plates of glass and mica." I 

 shall have to name an experiment or two presently which will 

 show, in a striking manner, the influence of the "calorific rays" 

 on metallic plates. I regard those emanations, which are the 

 acting ones in all our photographic operations, as possessing 

 powers of the most energetic and extraordinary kinds ; powers 

 which are in constant activity, decomposing and recomposing, 

 maintaining the conditions of growth and decay, of vitality and 

 corruption; indeed, effecting that mighty system of change, 

 which is the order of the visible creation, and these may be the 

 radiations by which all the spectral images of Moser are pro- 

 duced. Sir John Herschel* was inclined to attribute these 

 phfenomena to a class of rays ranging above the red rays of 

 the spectrum, and to which he gave the name of " parathermic 

 rays." The evidence, however, of the existence of these rays, 

 as a distinct class, is not sufficiently clear even to satisfy the 

 mind of this talented and most indefatigable observer, to whom 

 we are indebted, more than to any other person, for our know- 

 ledge of the conditions of the solar spectrum. 



At the Cork meeting of the British Association I commu- 

 nicated the results of some experiments made with the pris- 

 matic spectrum itself, which is the only way in which, as far 

 as I am aware, we can arrive at any satisfactory determina- 

 tion on the point in question. 



A condensed pris- 

 matic spectrum was, by 

 means of a good heli- 

 ostat, maintained in 

 one place upon a very 

 highly polished copper 

 plate for three hours. 

 At the expiration of 

 that time, the plate was 

 exposed to the action 

 of mercurial vapour. 

 This vapour was con- 

 densed over the whole 

 plate, but in very dif- 

 ferent proportions be- 

 yond and within the 

 limits of the spectrum. 

 The space occupied by 

 the luminous image was 

 evidently protected 

 from that influence 

 which disposed the other parts of the plate to condense the 

 * Philosophical Magazine, July 1843, (pres. vol.) p. 510. 



