330 On some early Experiments in Photography. 



decessovj he was a pioneer in the philosophy of Hght ; and, as 

 we have seen, by a single photographic experiment, overlooked 

 hitherto by us all, has shown a perfect analogy between the un- 

 dulations of the visible and invisible rays. Had he happened 

 to head his chapter, as Wedgwood does, "On a method of taking 

 Pictures by the agency of Light on Nitrate of Silver," his name 

 and place would have been duly marked ; but because theory, 

 and not experiment, was the great point before him, the philo- 

 sophical photographer is overlooked by the practical one. 



Dr. Young's propositions are, that radiant light consists in 

 undulations of the lummiferous pether, that light differs from 

 heat only in the frequency of its undulations, that undulations 

 less frequent than those of light produce heat, and that undula- 

 tions more frequent than those of light produce chemical and 

 photographic action, — all proved by experiments. 



You close your own ' Researches on Light ' by proposing the 

 following questions as of the greatest importance for future inves- 

 tigation : — " Is energia absorbed by material bodies ? Does it 

 influence their internal constitution ? Is it radiated from bodies 

 in the dark, or at all concerned in the production of any of those 

 changes which have been attributed to dark rays V 



Dr. Young's hypothesis seems to anticipate your questions, 

 and almost to answer them in their order. He says, " All mate- 

 rial bodies have an attraction for the (etherial medium, — by means 

 of which it is accumulated within their substance, — and for a 

 small space around them — in a state of greater density, but not 

 of greater elasticity." (Bakerian Lecture, 180L) Hence he con- 

 siders material bodies to have within them latent light, latent 

 heat, and latent chemical force, or " energia " (which is, in his 

 opinion, a particular condition of the retherial medium) ; that the 

 luminous, calorific, and chemical phsenomena are exhibited under 

 two modifications, — the vibratory or permanent, and the undula- 

 tory or transient state ; and that the forces which produce these 

 several effects differ from each other only in the frequency of 

 their undulations or vibrations. 



Such are the conclusions at which the all-inquir^ig Dr. Young 

 arrived in 1801, on a subject which in 1850 is proposed for our 

 investigation. Well may Admiral Smyth say, " How strange it 

 is that we are still but half acquainted with the results of his 

 powerful mind!" 



Of course I shall not quarrel with you if you do not accept 

 his conclusions, totidem verbis, because, as I am aware, you see 

 reasons for believing that light, or that agent which affects the 

 oi'gans of sight, is broadly distinguished from those rays which 

 bring heat from its solar source, and both of these classes from 

 those which produce, in the constitution of bodies, those singular 



