182 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
found in them matter of sufficient interest to render their longer suppression unad- 
visable, 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 demand, 
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 ac- 
count 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 Fraunhofer, there mentioned ; the area of the sec- 
tion of the incident sunbeam = T54 square inch, and the dimensions of the principal 
elements of the luminous spectrum, identical with those recorded in §. 70, so that 
the following results, when numerically stated (in measures of which the unit is one- 
thirtieth of an inch), will be comparable with those previously described. To spare 
reference, however, it may be here mentioned that the diameter of the sun’s image in 
the focus of the achromatic lens used is 7‘20 of such thirtieths ; and that the extent 
of the visible spectrum corrected for the sun’s semidiameter at either end, equals 
53*92 thirtieths, of which 13*30 are considered as reckoned negatively to the extreme 
visible red from a fiducial point or centre corresponding to the mean yellow ray ; and 
40*62 positively, from the same centre to the terminal violet, both as seen through a 
certain standard blue glass, which lets both extremes pass freely and insulates the 
mean yellow with considerable precision. The correction for the sun’s semidiameter 
has been applied in what follows to all measures up to terminations of spectra , unless 
where the contrary is expressed. Maxima and minima of action, and neutral points 
neither require nor admit this correction. 
Guaiacum. 
151. A solution of this resin in alcohol, spread evenly on paper, gives a nearly co- 
lourless ground. A slip of this paper exposed to the spectrum is speedily impressed 
with a fine blue streak over the region of the violet rays, and far beyond, as described 
in Art. 92. If the paper during this action be carefully defended from extraneous 
light, this is the only perceptible effect ; but if dispersed light be admitted, the general 
ground of the paper is turned to a pale brownish green, with exception of that por- 
tion on which the less refrangible rays fall, which, by their agency, is defended from 
the action of the dispersed light and preserves its whiteness, as in the case of the ar- 
gentine paper described in Art. 60. The spectrum, therefore, ultimately impressed, 
