12 Dr. Gardner on the Action of Yellow 



37. Upon projecting these numbers, which, although not 

 rigorously correct, are very good approximations, the unity of 

 the active agent will be more strikingly exhibited. Let the 

 axis of abscissas be divided into intervals corresponding to 

 Fraunhofer's coloured spaces, and the positions of the mean 

 places of the dark lines be marked from Mr. Powell's recent 

 work on dispersion. The ordinates are from the preceding 

 table; Fraunhofer's estimates are indicated by a bold line, 

 Dr. Draper's by dots, and my own by an interrupted line. 



Fig. 1. 



Had more points in these figures been determined, there is 

 no doubt they would have coincided precisely. It is not to 

 be forgotten that these results were obtained in places many 

 hundred miles apart. They determine, what hitherto has only 

 been conjectured, that the greening of plants and decomposi- 

 tion of carbonic acid are produced by the same agent, which 

 is also the active imponderable in producing vision, a pheno- 

 menon in no way similar, as suggested by M. Moser, to the 

 change of Daguerre's plate, which is a tithonic action. 



The dependence of the depth of green colour in foliage 

 upon brilliant light is also shown. The statements of tra- 

 vellers, in regard to tropical vegetation, confirms this conclu- 

 sion. 



38. Chlorophyl, the body generated in the yellow leaflets 

 of plants, raised in darkness by the action of light, is a hydro- 

 carbon of the nature of wax. Whether it be produced by 



