Action of the Solar Spectrum on Vegetable Colours. 107 
d z is established, is, I think, essential to complete the general 
theory of hydrodynamics, and deserves the particular atten- 
tion of mathematicians engaged on this subject. 
The kind of motion spoken of by Mr. Stokes at the top of 
p. 56, is sufficiently defined by its being common as to magni- 
tude and direction to all the parts of the fluid. Such motion, 
I said, must be considered to be eliminated from the hydro- 
dynamical equations. For if it were included in them, the 
terms relating to it would separate themselves from the terms 
relating to fluid motion, being in fact the same as if the fluid 
were supposed to be solid. 
I understand Mr. Stokes to argue in the concluding para- 
graph of his reply, that if wd2+vdy+wd s be an exact 
differential at one instant, when powers of the velocity above the 
first are omitted, it will be so always, the smallness of the 
velocity being the reason for omitting the higher powers, but 
not the reason that the differential is exact. If this is what 
is meant I quite agree with it. It is a very different thing to 
argue thatudx +vdy+wd-z is an exact differential because 
the motion is small. I have already pointed out the fallacy 
of this argument. 
XVII. On the Action of the Rays of the Solar Spectrum on Vege- 
table Colours, and on some new Photographic Processes. By 
Sir Joun F. W. HeErscuHe1, Bart., K.H., F.R.S. 
[Continued from p. 21.] 
Turmenic.— Further proofs of the continuation of the visible 
Prismatic Spectrum beyond the extreme Violet, 
185. (THE action of light on paper coloured with the alco- 
holic tincture of turmeric is but feeble. If long 
continued, however, it is whitened in the region of the blue 
and violet rays, from +10 to + 43, or thereabouts, the maxi- 
mum being at + 23°5. The paper browned by carbonate of 
soda is somewhat more sensitive, especially when wet, in which 
case an abruptly terminated action is perceptible in the red 
region, giving rise to adouble maximum at —10:0 and + 22:5, 
with an intermediate minimum at —4°0. I should not have 
_ thought it necessary, however, to mention this paper, but on 
° 
account of a remarkable peculiarity in its reflective power, in 
virtue of which it renders very plainly visible a prolongation 
of the spectrum beyond the extreme violet, in the region of 
what I have termed in my last paper,the Lavender rays. As the 
experiment is easily made, and affords a ready method of ren- 
dering visible this part of the spectrum, I shall describe, with 
