DECOMPOSITION OF THE RETINA AND OTHER CARDON COMPOUNDS. 75 



cific colours is involved, remains as yet undetermined ; we cannot say whether the 

 quality by which we are led to impute to a given beam a yellow colour is the same 

 quality which is involved in this decomposition. Whether, in short, yellow light, be- 

 cause it is yellow light, produces this change. Would any other coloured ray, such as 

 a blue, if its intensity were sufficiently elevated, produce the same result ! For we can 

 imagine a blue light so to be re-enforced as to possess the same intrinsic brilliancy or 

 illuminating power as a yellow. Under such a change, would its decomposing action 

 also be exalted ? 



278. Prismatic experiments serve to show (Ap., 782) that the rapidity of decompo- 

 sition follows very closely the order of illuminating power. And this result affords an 

 argument, imperfect and feeble it is true, that an affirmative answer will be hereafter 

 given to that question. 



279. But there are other reflections which naturally arise, and tend to an opposite 

 result. There seems to be a general relation, though the details of it have not yet been 

 traced, between rays of a particular refrangibility and ponderable substances of a par- 

 ticular kind. Thus, in the case of most of the salts of silver, the point of maximum 

 action falls in the violet ray. In the same way the question naturally arises, Does the 

 point for the maximum action on carbon compounds fall in the yellow space, and the 

 yellow, for that reason, become the active ray in decomposing carbonic acid, and giv- 

 ing a green colour to leaves 1 Is it for this cause, also, that, received into the eye, the 

 yellow ray impresses us with the greatest illuminating power I It would be a beauti- 

 ful result of these researches to co-ordinate phenomena apparently so widely apart as 

 the formation of chlorophyl in a leaf and the regulated destruction of the retina in the 

 chamber of the human eye in producing the phenomena of vision. In nature there 

 are many results which are apparently equally distinct, and which the progress of 

 knowledge has shown are intimately allied. That to our organs of vision yellow light 

 is the most brilliant, arises from the incidental circumstance that it is a carbonaceous 

 compound of which the changing nervous expansion is constructed. Had it been pos- 

 sible for Nature to have formed a retina in which a salt of silver formed the basis, the 

 maximum of brilliancy of light would have shifted, and the blues would have been 

 among the brightest rays. Is it in the optical peculiarities of the carbon atom that all 

 our ideas of harmony among colours and beauty of external objects have arisen I 



280. Experimental science will probably before long trace a close connexion between 

 the physical properties of atoms and the physical properties of light. It will show that 

 molecules of a given weight can be moved most easily by ethereal waves of a given 

 length, as a stretched string is thrown into vibration by atmospheric undulations of 

 proper dimensions; that the transverse vibrations of the ethereal particles can agitate 

 in a corresponding way ponderable atoms of a proper magnitude and constitution. We 

 shall then have no difficulty in understanding how it was that among metallic sub- 

 stances, those first detected to be changed by light, such as silver, gold, mercury, lead, 

 have all high atomic weights; and such as sodium and potassium, the atomic weights 

 of which are low, appeared to be less changeable. 



