226 Royal Society. 



coloured by various vegetable juices, and afterwards washed with 

 various solutions. The action of solar light he found to be exceed- 

 ingly various, both as regards its total intensity and the distribution 

 of the active rays over the spectrum. He observed, however, that 

 the following peculiarities obtain almost universally in the species 

 of action exerted on vegetable colours. 



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. 



Secondly, the action of the spectrum is confined, or nearly so, to 

 the region of it occupied by the luminous rays, as contra-distinguished 

 both from the so-called chemical rays beyond the violet, (which act 

 with chief energy on argentine compounds, but are here for the 

 most part ineffective,) on the one hand, and on the other, from the 

 thermic rays beyond the red, which appear to be totally ineffective. 

 Indeed, the author has not hitherto met with any instance of the 

 extension of this description of photographic action on vegetable 

 colours beyond, or even quite up to the extreme red. 



Besides these, the author also observed that the rays which are 

 effective in destroying a given tint, are, in a great many cases, those 

 whose union produces a colour complementary to the tint destroyed, 

 or at least one belonging to that class of colours to which such com- 

 plementary tint may be referred. Yellows tending towards orange, 

 for example, are destroyed with more energy by the blue rays ; blues 

 by the red, orange and yellow rays ; purples and pinks by yellow 

 and green rays. These phenomena may be regarded as separating 

 the luminous rays by a broadly defined line of chemical distinction 

 from the non-luminous ; but whether they act as such, or in virtue 

 of some peculiar chemical quality of the heat which accompanies 

 them as heat, is a point which the author considers his experiments 

 on guaiacum as leaving rather equivocal. In the latter alternative, 

 he observes, chemists must henceforward recognize, in heat from dif- 

 ferent sources, differences not simply of intensity, but also of quality ; 

 that is to say, not merely as regards the strictly chemical changes it 

 is capable of effecting in ingredients subjected to its influence. 



One of the most remarkable results of this inquiry has been the 

 discovery of a process, circumstantially described by the author, by 

 which paper washed over with a solution of ammonio-citrate of iron, 

 dried, and then washed over with a solution of ferro-sesquicyanuret 

 of potassium, is rendered capable of receiving w^th great rapidity a 

 positive photographic image ; and another in which a picture nega- 

 tively impressed on a paper washed with the former of these solu- 

 tions, but which originally is faint and sometimes scarcely percep- 

 tible, is immediately called forth on being washed over with a 

 neutral solution of gold. The picture does not at once acquire 

 its full intensity, but darkens with great rapidity up to a certain 

 point, when the resulting photograph attains a sharpness and per- 



