OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
183 
consists of two portions similar 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 inten- 
sity at O'O, and extending from — 11*9 (corrected for the sun’s semidiameter) to 
-f- 120, or thereabouts, at which point the character of the action changes, and a 
blue, of a somewhat smoky grey cast, commences, which attains a maximum at 
-j- 40’0, thence degrades to an intermediate minimum at + 47*0, attains a second 
and much stronger maximum at + 6 TO, and ceases at 72*4. 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 XV. fig. 1, in which the abscissae 
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 correspond- 
ing point, as estimated from the amount of colour induced or prevented. In this 
type the portion corresponding to the less refrangible rays is represented by negative 
values of the ordinate agreeably to Art. 93, where it is shown that these rays not 
only prevent the blue colour from being produced by the more refrangible ones, but 
destroy it when so produced. Another specimen gave the following dimensions : 
Y a = — 1 1*4, Y b — — 9‘5, Y c = -j- 30 - 0, Y d = -f- 6T0, Y e — -f- 80*4, and this is 
the greatest extent of action I have hitherto observed. 
152. A portion of the same paper was exposed, dry, to an atmosphere of chlorine 
considerably diluted with common air, which imparted to it a pale, dirty, greenish 
yellow hue. Being thence transferred immediately to the spectrum, the result was 
not a little remarkable. The whole spectrum, the green excepted, was impressed in 
faint tints nearly corresponding to the natural ones. The red was evident — the 
yellow dilute and nearly white — the blue a fine sky-blue, while beyond the violet 
succeeded a train of somewhat greenish darkness. These tints proved fugitive, and 
in twenty-four hours were nearly obliterated. 
153. When paper fresh washed with tincture of guaiacum and still wet is exposed 
to chlorine, it instantly acquires a fine and full Prussian blue colour, which however 
passes speedily to brown if the action be prolonged. The colour is difficult to pre- 
serve in its full intensity, and fades considerably in drying, becoming at the same time 
somewhat greenish. Exposed wet to the spectrum, it is found to have become much 
more sensitive, and is immediately attacked with great energy by the red rays, which 
destroy the blue colour, converting it to a brownish or reddish yellow. The action 
extends rapidly up the spectrum as far as the extreme violet, in which ray, however, 
the tint impressed or left undestroyed passes to a hue partaking of violet, and indi- 
cating by the change what ought probably to be regarded as a neutral point at 
-f- 12*0. The impressed spectrum (corrected for semidiameter) commences at a, fig. 
2, at — 13*4 ; the maximum b of the positive action occurs at — 9’0, the neutral point 
c at + 12’0, the maximum d of negative action at + 33‘0, and the sensible termi- 
nation e of the impression at + 60'0. 
154. The action of gaseous chlorine is too energetic to be easily arrested at the 
