133 
of Edinburgh, Session 1872-73. 
These experiments, then, prove that the transmission of light 
through the blood-vessels is not necessary for the production of the 
crimson flush, and that a long rest be given to the eye to per- 
ceive the phenomenon described by Professor Tait, and that the 
former depends upon the latter effect. 
In the experiments last described the whole of the retina was 
affected. There is still one point that requires explanation. How 
is it that either a very powerful or a very feeble light is the most 
potent, either a gas flame or diffuse light that has passed through 
several folds of paper ? At first this seems to militate against the 
identity of the two phenomena, but a little consideration explains 
- difficulty. First, if the light be very bright, e.g., a gas flame, the 
red will certainly have a greater tendency to appear, but it seems 
a 'priori likely that the other colours will also soon become apparent. 
Thus we should expect with a powerful flame to see a very intense 
redness, lasting a very short time. Second, if the light be very 
feeble, e.g., diffuse light passing through paper. Here it is not 
likely that we should get so brilliant a red, but it is certainly very 
probable that it will be much longer before the other colours become 
sensible, since they are so feeble. We should expect then, in this 
case, to have a less powerful red lasting a longer time, but with the 
gas flame a strikingly brilliant flush, lasting a very short time. 
Again, with a medium light the green and blue colours would be 
added rapidly, and the crimson flush would not be powerful enough 
to be conspicuous in that short time. I may say that in every point 
this agrees exactly with the appearances as they are really seen. 
4. A Theory of Volcanic Eruptions. By Daniel Vaughan. 
From researches which have much engaged my attention for 
nearly twenty years, I am convinced that silica performs^ very im- 
portant part, not only in the formation of the earth’s crust, but also 
in leading to violent subterranean movements. The low specific 
gravity of silicic acid, and of the rocks in which it predominates, 
would (if much of the internal earth were fluid) give rise to certain 
results, which I traced in an essay published in 1856, and also in a 
paper which was brought before the British Association for the 
Advancement of Science in 1861. In the latter, I have given 
