6 Sir J. F. W. Herschel on the Action of the Rays 
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 exposure 
to the concentrated spectrum for many hours, to clear sun- 
shine 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 experi- 
ments, 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 em~ 
ployed, 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 per- 
ception of characters which might serve as guides in an intri- 
cate inquiry, and afford hints for the conduct of future ex- 
periment. 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 connexion; nor 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 found in them 
matter of sufficient interest to render their longer suppression 
unadvisable, and to induce others more favourably situated as 
to climate, to prosecute the subject. 
150. The materials operated on in these experiments have 
been for the most part the juices of the flowers or leaves of 
plants, expressed, either simply, or with addition of alcohol, 
or under the influence of other chemical reagents. Some 
few resinous and dyeing substances have also been subjected 
to experiment, but with less perseverance than the obvious 
practical importance of this branch of the subject might de- 
mand, except in the case of guaiacum, whose relations to light, 
heat, and chemical agents are exceedingly remarkable and 
instructive, for which reason, as well as because some of these 
relations have been treated of in my former paper, I shall 
commence the account of my later experiments with those 
made on this substance. But in the first place it is necessary to 
state that the apparatus used for forming, concentrating, and 
fixing the spectrum, was the same with that described in Art. 
67. of that paper; the prism being that of flint-glass by Fraun- 
hofer, there mentioned ; the area of the section of the incident 
sunbeam = 1°54 square inch, and the dimensions of the prin- 
cipal elements of the Juminous spectrum, identical with those 
recorded in §, 70, so that the following results, when numeri+ 
cally stated (in measures of which the unit is one-thirtieth of 
an inch), will be comparable with those previously described. 
To spare reference, howeyer, it may be here mentioned that 
the diameter of the sun’s image in the focus of the achro- 
