116 CORRELATION OF PHYSICAL FORCES. 



I have used the term light, and affected by light, in speak 

 ing of photographic effects ; but, though the phenomena de 

 rived their name from light, it has been doubted by many 

 competent investigators whether the phenomena of photo 

 graphy are not mainly dependent upon a separate agent ac 

 companying light, rather than upon light itself. It is, indeed, 

 difficult not to believe that a picture, taken in the focus of a 

 camera-obscura, and which represents to the eye all the gra 

 dations of light and shade shown by the original luminous 

 image, is not an effect of light ; certain it is, however, that 

 the different coloured rays exercise different actions upon va 

 rious chemical compounds, and that the effects on many, per 

 haps on most of them, are not proportionate in intensity to 

 the effects upon the visual organs. Those effects, however, 

 appear to be more of degree than of specific difference ; and, 

 without pronouncing myself positively upon the question, 

 hitherto so little examined, I think it will be safer to regard 

 the action on photographic compounds as resulting from a 

 function of light. So viewing it, we get light as an initia 

 ting force, capable of producing, mediately or immediately, 

 the other modes of force. Thus, it immediately produces 

 chemical action ; and having this, we at once acquire a means 

 of producing the others. At my Lectures in 1843, I showed 

 an experiment by which the production of all the other modes 

 of force by light is exhibited : I may here shortly describe it. 

 A prepared daguerreotype plate is enclosed in a box filled 

 with water, having a glass front with a shutter over it. Be 

 tween this glass and the plate is a gridiron of silver wire ; 

 the plate is connected with one extremity of a galvanometer 

 coil, and the gridiron of wire with one extremity of a Bre- 

 guet s helix an elegant instrument, formed by a coil of two 

 metals, the unequal expansion of which indicates slight 

 changes in temperature the other extremities of the galva 

 nometer and helix are connected by a wire, and the needles 

 brought to zero. As soon as a beam of either daylight or 



