OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
191 
a much feebler whitening 1 becomes sensible in the red, and a trace of it also beyond 
the violet into the “ lavender” rays. In this state the type of the impressed spectrum 
(in an experiment made on the 7th of April in the present year) was as in fig. 4, in- 
dicating three obsolete maxima c, cl, e, and a very sudden diminution of the action at 
b, f, the dimensions being as follows : Y a — — 94, Y b = + 7‘1, Y c = + 125, 
Y d = + 23 - 5, Y e = + 34 - 0, Y f — + 41 - 4, Yg = -f- 59'7- The paper thus impressed 
was again re-examined on the 2nd of May, or after twenty-five days, during which 
interval it had been exposed to free air, but only to feeble and dispersed occasional 
lights. It was found to have undergone a remarkable change, two distinct white 
spots having become insulated, or nearly so, at the very extremities of the impressed 
spectrum, the three maxima above indicated having also become much more distinct, 
and two new, subordinate ones, having begun to show themselves in the faint traces 
connecting the spots above mentioned witli the main impression. The type of the 
spectrum in this state was as represented in fig. 5, and the places of the several max- 
ima being as follows: — 1st, — 10*0; 2nd, — 0’5 ; 3rd, -j- 12’0 ; 4th, -f- 29’0 ; 5th, 
+ 40*0 ; 6th, + 50 : : ; 7th, + 61*0. The terminal spot at the red extremity was nearly 
equal in diameter to the sun’s image ; that at the least refracted end, corresponding 
in place to rays much beyond the last violet, was smaller, but perfectly distinct ; 
and as it constitutes the only instance I have yet encountered of a definite ray in this 
region of the spectrum*, I have been thus particular in describing the phenomenon. 
177- Common ten-weeks Stock, Mathiola annua. — The colour imparted by the pe- 
tals of the double variety of this flower'f- to alcohol (at least when spread on paper, for 
it is in great measure dormant in the liquid tincture) is a rich and florid rose-red, 
varying, however, from a fiery tint almost amounting to scarlet, on the one hand, to 
a somewhat crimson or slightly purplish red on the other, according to the acci- 
dents of its preparation, or the paper used. When fresh prepared it is considerably 
sensitive, an hour or two of exposure to sunshine being sufficient to produce a sen- 
sible discoloration, and two or three days entirely to whiten it. This quality is 
greatly deteriorated by keeping, but papers prepared with it even after eight or ten 
months, still with patience yield extremely beautiful photographs, several specimens 
of which in various states of the tincture are submitted for inspection to the meeting. 
Exposed to the spectrum, the rays chiefly active in operating the discoloration are 
found to be those extending from the yellow to the less refrangible red, beyond which 
rays the action terminates abruptly. Above the yellow it degrades rapidly to a 
minimum in the blue, beyond which it recovers somewhat, and attains a second but 
much feebler maximum in the violet rays. 
* Since this was written, other cases, extremely remarkable, among the argentine preparations, have presented 
themselves. See Art. 214. 
t That imparted by the single flowers is very much less sensitive, as is also that of the dull red or purplish 
variety, whether double or single. The most florid red double flowers in the height of their flowering, yield 
the best colour. 
