of the Solar Spectrum on Vegetable Colours. rd 
matic 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 con- 
sidered 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 ter- 
minal 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 éerminations of spectra, unless where the con- 
trary is expressed. Maxima and minima of action, and neu- 
tral points neither require nor admit this correction. 
Guaiacum. 
151. A solution of this resin in alcohol, spread evenly on 
paper, gives a nearly colourless ground. A slip of this paper 
exposed to the spectrum is speedily impressed with a fine blue 
streak over the regiun 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 per- 
ceptible effect ; but if dispersed light be admitted, the general 
ground of the paper is turned to a pale brownish green, with 
exception of that portion 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 argentine paper described in Art. 60. The spectrum, 
therefore, ultimately impressed, consists of two portions si- 
milar to those described in Art. 93, and of nearly the same 
extent, that is to say, a white or pale yellowish portion 
having its maximum of intensity at 0°0, and extending from 
— 11°9 (corrected for the sun’s semidiameter) to + 12:0, 
or thereabouts, at which point the character of the action 
changes, and a blue, of a somewhat smoky gray cast, com- 
mences, which attains a maximum at +40°0, thence degrades 
to an intermediate minimum at + 47:0, attains a second and 
much stronger maximum at +61:0, and ceases at 724. The 
precise numbers vary materially in different specimens and 
with the length of exposure. ‘The type of this spectrum, of 
its natural length, is represented in Plate I. fig. 1, in which 
the abscissee being measured along the length of the spectrum, 
from the fiducial centre Y both ways, the ordinates express 
the intensities of photographic action at each corresponding 
point, as estimated from the amount of colour induced or pre- 
vented. In this type the portion corresponding to the less 
refrangible rays is represented by negative values of the or- 
