OF THE SPECTRUM ON VEGETABLE JUICES. 
113 
influence of the yellow and green rays upon vegetable substances, an influence which 
seems to be unconnected with heat, since the darkening generally was least under 
the red rays, and immediately below them, where the calorific rays are most abun- 
dant. I found that the action of the rays of mean refrangibility on vegetable juices 
was much increased by the addition of a little sulphuric acid. In most cases this 
powerful influence was simultaneous with the appearance of two dark spots below 
the visible spectrum, sometimes sharply defined, sometimes in the centre of a disc of 
a paler hue; and in some instances the disc was seen without spots, though it rarely 
happened that they did not appear under one or other of these forms. 
The following are some of the cases in which the simultaneous effect was produced : 
for example, paper washed with juice of the petals of Globe amaranthus in distilled 
water, on exposure to the spectrum acquired a delicate pink tint, which was soon 
bleached to whiteness, from the upper edge of the green to the end of the lavender 
rays, while at the same time a perfectly circular spot of equal whiteness was seen 
under the red rays, and a little way below them, which had the appearance of being 
an image of the sun. After more washing with the juice, the two bleached parts 
were united by a long white neck, which speedily vanished, and was succeeded by a 
dark crimson image, whose greatest intensity of colour was under the yellow rays. 
At some distance below the red rays, two crimson spots were strongly marked, 
especially the uppermost, both surrounded by a paler halo (fig. 4.). 
The juice of the petals of pale blue Plumbago auriculata in distilled water imparted 
its tint to writing-paper, which, after exposure to the action of diffused light, acquired 
a pale yellowish green hue. The part under the lavender and violet rays of the 
spectrum, repeatedly washed with the juice, assumed a pale brown colour : the indigo 
rays seemed to have no effect, although from their lowest edge to the distance of half 
the length of the spectrum below the red rays, a lavender blue image was formed. 
Under the orange rays a minute indigo-coloured spot appeared, and also a larger 
spot of the same colour under the yellow, which were soon blended into one, forming* 
a single oblong figure of maximum intensity, surrounded by a halo of paler indigo 
(fig. 5.). An insulated disc of the same colour as the halo, with two dark spots in its 
centre, appeared at some distance below the red rays. The juice of this plant is easily 
decomposed ; by the addition of sulphuric acid it became a delicate pinkish grey, which 
was also the colour of paper wetted with it, while under the spectrum a pale yellow 
figure of considerable width was impressed, extending under the whole of the visible 
rays, and soon after a deep yellow image with a paler halo was seen in its centre. 
The halo or paler part of the image lay between the red and the end of the lavender 
rays, while the deep yellow, or perhaps more properly the yellow-brown figure, ex- 
tended under the yellow-green and blue rays, the maximum of breadth and of colour 
lying in the yellow and green. Two insulated spots of the same yellowish-brown in 
the centre of a yellow disc were formed below the visible spectrum, and the whole 
surface soon became bright yellow from the action of the scattered light, conse- 
Q 2 
