OF THE SPECTRUM ON VEGETABLE JUICES. 
119 
The darkest part lay between the lowest edge of the blue and the highest extremity 
of the lavender rays: that under the lavender was bright red, while the part under 
the violet, indigo and blue, was orange-coloured. From the blue to the end of the 
red was deep yellow, and below the red rays two insulated spots of the same deep 
yellow were formed, though somewhat blended together. On the juice of Plumbago 
auriculata the lavender and violet rays produced a pale brown image; the indigo 
ravs had no effect, while all the rest of the image under the mean and least refran- 
gible rays was blue and indigo; some of the juice having remained till next day, the 
moisture had evaporated from it, leaving a yellowish-brown sediment in the bottom 
of the cup, surrounded by a border of indigo, the very same colours that were sepa- 
rated by the solar rays and impressed upon paper the day before, showing a striking 
analogy between the action of the two ends of the spectrum and that of the poles of 
a galvanic battery. There was a green ring between the brown and the blue where 
the two colours were blended, and this also was visible in the greenish-yellow tint of 
the general ground of the paper after exposure to light. I now expected to find that 
paper washed with the brown sediment, dissolved in distilled water, would be im- 
pressed with the brown part of the image only, and this was the case, with the ex- 
ception of some slight indication of brown at the place of the spots. There was not 
enough of the blue pure to enable me to obtain the indigo part of the figure alone. 
The variation in the width of the images impressed by the same spectrum on dif- 
ferent vegetable substances was remarkable ; on the juice of the yellow thistle already 
mentioned, the image was but a mere line, nor was it much broader on the juice of 
Plumbago : it was wider on the juice of the dahlia, but the addition of a drop of sul- 
phuric acid increased the width greatly, as in the images formed on the juice of beet- 
root, the petals of dahlia, balsam and others. Possibly the cause of this may have 
been the dispersion of the rays by the acid, which was particularly great in the most 
luminous rays of the spectrum, though in various instances, where acid was not used, 
the action of the spectrum seemed to extend laterally beyond its visible edges. This 
occurred especially when a surface of slightly browned nitrate of silver was washed 
with vegetable juices or other liquids. Under these circumstances the image fre- 
quently consisted of a succession of borders of various breadth and intensity sur- 
rounding a central oblong nucleus, resembling the section of a bulbous root cut lon- 
gitudinally and exhibiting its coatings. For example, a slightly sunned surface of 
nitrate of silver, washed with the juice of the young green walnut in a strong solu- 
tion of common salt became lilac, though the liquid itself was brownish ; but under 
the spectrum an olive-brown image was formed from the upper edge of the orange 
rays to a considerable distance beyond the violet, where it tapered to nearly a point. 
After two or three washings with the liquid, the internal part was changed to a paler 
tint from the indigo rays to below the red. The part projecting down from the red 
was much narrower than the rest of the figure and ended in a curve. Then a dark 
and almost elliptical figure was formed within the pale part inclosing part of the 
MDCCCXLVI. R 
