108 Sir J. F. W. Herschel on the Action of the Rays 
some minuteness, the appearances which presented themselves 
in my experiments, and which seem to place the real existence 
of those heretofore undescribed ]uminous rays beyond all rea- 
sonable objection, should any doubt have arisen as to the in- 
terpretation of the phenomenon described in my former paper 
(Art. 59.). : 
186. Paper stained with tincture of turmeric is of a brilliant 
yellow colour, and in consequence, the spectrum thrown on 
it, if exposed in the open daylight, is considerably affected in 
its apparent colours, the blue portion appearing violet, and 
the violet very pale and faint; but beyond the region occupied 
by the violet rays is distinctly to be seen a faint prolongation 
of the spectrum, terminated laterally, like the rest of it, by 
strait and sharp outlines, and which in this case affects the eye 
with the sensation of 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 results being as follows (see Plate I. fig. 6.) :— 
Length of the spectrum Y L from the fiducial ) Part. 
point Y to the visible termination L, as seen (with 
the naked eye) on the turmeric paper; corrected Bane 
for G’S SEMIDIAMEtEL sosccacsdasssenwccsnrancesseesan 
Length Y V from the same fiducial point to the 
visible termination, as similarly seen when pro- - = 40'4 
jected on white Paper seseeeeseeceececerececeneeenen ees 
Prolongation rendered visible by projection of | _ 16°2 
the spectrum ON tUTMETIC PAPEL ...cceseeseeceeeeeees 
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 halo-light 
passes through the sun, which in a meridional position of that 
luminary might give rise to a perceptible prolongation both up- 
wards 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 ornear 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 material, as 
to its reflective powers. In particular, a certain paper (No. 
1055.) coloured with the juice of Chryseis 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. (cor- 
recting 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 tur- 
