393 



rimental processes employed in his investigation ; and points out 

 several circumstances which require to be attended to in order to 

 ensure success. 



June 16, 1842. 



SIR JOHN W. LUBBOCK, Bart., V.P. and Treas., in the Chair. 

 The following papers were read, viz. — 



1. " On the Action of the Rays of the Solar Spectrum on Vegetable 

 Colours." By Sir John Frederick William Herschel, Bart., K.H., 

 F.R.S. 



The author, having prosecuted the inquiry, the first steps of which 

 he communicated in a paper read to the Royal Society in February 

 1 840, relating to the effects of the solar spectrum on the colouring 

 matter of the Viola tricolor, and on the resin of guaiacum, re- 

 lates, in the present paper, the results of an extensive series of simi- 

 lar experiments, both on those substances, and also on a great number 

 of vegetable colours, derived from the petals of flowers, and the leaves 

 of various plants. In the case of the destruction of colour of the pre- 

 parations of guaiacum, which takes place by the action of heat, as 

 well as by the more refrangible rays of light, he ascertained that 

 although the non-luminous thermic rays produce an effect, in as far 

 as they communicate heat, they are yet incapable of effecting that 

 peculiar chemical change which other rays, much less copiously en- 

 dowed with heating power, produce in the same experiment. He 

 also found that the discoloration produced by the less refrangible 

 rays is much accelerated by the application of artificial terrestrial 

 heat, whether communicated by conduction or by radiation ; while, 

 on the other hand, it is in no degree promoted by the purely ther- 

 mic rays beyond the spectrum, acting under precisely similar cir- 

 cumstances, and in an equal degree of condensation. The author 

 proceeds to describe, in great detail, the photographic effects pro- 

 duced on papers coloured by various vegetable juices, and after- 

 wards washed with solutions of particular salts ; and gives a minute 

 account of the manipulations he employed for the purpose of im- 

 parting to paper the greatest degree of sensitiveness to the action of 

 solar light. This action he found to be exceedingly various, both as 

 regards its total intensity and the distribution of the active rays over 

 the spectrum. He observed, however, that the following peculiar- 

 ities obtain almost universally in the species of action exerted. 



First, the action is positive ; that is to say, light destroys colour, 

 either totally, or leaving a residual tint, on which it has no further, 

 or a very much slower action ; thus effecting a sort of chromatic ana- 

 lysis, in which two distinct elements of colour are separated, by de- 

 stroying the one and leaving the other outstanding. The older the 

 paper, or the tincture with which it is stained, the greater is the 

 amount of this residual tint. 



