[ '81 ] 
XII. On the Action of the Rays of the Solar Spectrum on Vegetable Colours , and on 
some new Photographic Processes. By Sir John F. W. Herschel, Bart. K.H. F.R.S. 
Received June 15, — Read June 16, 1842. 
i49* In my paper on the “ Chemical Action of the Solar Spectrum on prepara- 
tions of Silver and other substances,” read to the Royal Society in February 1840 , 
and of which the present communication is intended as a continuation or supplement, 
some experiments on the effect of the spectrum on the colouring - matter of the Viola 
tricolor , and on the resin of guaiacum are described, which the extreme deficiency 
of sunshine during the summer and autumn of the year 1839 prevented me from 
prosecuting efficiently up to the date of that communication. The ensuing year 1840 
was quite as remarkable for an excess of sunshine as its predecessor for the reverse. 
Unfortunately the derangements consequent on a change of residence prevented my 
availing myself of that most favourable conjuncture, and it was not till the autumn of 
that year that the inquiry could be resumed. From that time to the present date it 
has been prosecuted at intervals as the weather would allow, though owing to the 
almost unprecedented continuance of bad weather during the whole of the past sum- 
mer and autumn ( 1841 ), it has of late been almost wholly suspended'}'. In photogra- 
phic processes, where silver and other metals are used, the effect of light is so rapid 
that the state of the weather, as to gloom or sunshine, is of little moment. It is 
otherwise in the class of photographic actions now to be considered, in which expo- 
sure to the concentrated spectrum for many hours, to clear sunshine for several days, 
or to dispersed light for whole months, is requisite to bring on many of the effects 
described, and those some of the most curious. Moreover, in such experiments, 
when unduly prolonged by bad weather, the effects due to the action of light become 
mixed and confounded with those of spontaneous changes in the organic substances 
employed, arising from the influence of air, and especially of moisture, &c., and so 
give rise to contradictory conclusions, or at all events preclude definite results, and 
obscure the perception of characters which might serve as guides in an intricate in- 
quiry, and afford hints for the conduct of future experiment. It is owing to these 
causes that I am unable to present the results at which I have arrived, in any sort of 
regular or systematic connection ; rior should I have ventured to present them at all 
to the Royal Society, but in the hope that, desultory as they are, there may yet be 
* The paragraphs, for convenience of reference, are numbered in continuation of those of the previous paper 
referred to in the text. 
t This was written in April 1842, since which a repetition of the season of 1840 seems to have commenced. 
