OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
195 
a pale yellow colour. Comparative measures were carefully taken of the spectrum 
so prolonged^ and of the ordinary spectrum as seen projected on white paper, the re- 
sults being as follows (see fig. 6.) : — 
Length of the spectrum Y L from the fiducial point Y to the visible^ Parts, 
termination L, as seen (with the naked eye) on the turmeric paper; } = 566 
corrected for ©’s semidiameter 
Length Y V from the same fiducial point to the visible termination, 
as similarly seen when projected on white paper 
Prolongation rendered visible by projection of the spectrum on tur-~] 
meric paper J 
187- The day on which this experiment was first made (May 27, 1841) was serene 
and clear, but being aware that in certain states of the atmosphere a vertical beam 
of haio-light passes through the sun, which in a meridional position of that luminary 
might give rise to a perceptible prolongation, both upwards and downwards (though 
in fact no such prolongation was perceived at the red end), it was often repeated, and 
always with the same result, on subsequent occasions, whether the sun were on or 
near the meridian, or otherwise. Comparative trials, also with other yellow papers, 
fully satisfied me of the cause being traceable to a peculiarity in the colouring mate- 
rial, as to its reflective powers. In particular, a certain paper (No. 1055.) coloured 
with the juice of Chryse'is californica, whose tint was almost identical with that of the 
turmeric paper, only somewhat brighter, was tried, and the spectrum measured on 
this paper was found to terminate precisely at 44'0, i. e. (correcting for semidiameter) 
at 40*4, the very same as if white paper had been used. 
188. To test the matter yet more pointedly, a strip of turmeric paper was fixed on 
the Chryse'is paper, so that its edge should bisect the spectrum longitudinally from 
end to end, the preceding half of the sun’s lengthened image being received on the 
one paper, and the following half on the other. The papers thus arranged were so 
similar as hardly to be distinguished when simply laid in sunshine, but when illumi- 
nated by the spectrum, as above described, the half of it on the turmeric side was 
plainly seen to extend far beyond the other, as represented in fig. 6. 
189. Hitherto I have met with only one other coloured paper which possesses a 
similar character in respect of its reflective power, and that by no means in so high 
a degree. To prepare it, the alcoholic tincture of the dark purple dahlia must be 
alkalized by carbonate of soda. The mixture is vivid green, which is also, at first, the 
colour of paper stained with it. But this colour changes in about twenty-four hours 
to a fine yellow, a little inclining to orange, after which it is remarkably permanent, 
and very little sensible to photographic impression. On this, as on the turmeric paper, 
the prolongation of the spectrum appears as a pale yellow streak. And if such, rather 
than lavender or dove-colour, should be the true colorific character of these rays, we 
might almost be led to believe (from the evident reappearance of redness mingled with 
2 c 2 
