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 



