607 
of Edinburgh, Session 1871-72. 
3d, I suggest, as a possible explanation, but one which is more 
specially in the province of the physiologist than of the natural 
philosopher, that the retina (or the nerve cells connected with it?) 
partakes of sleep with the other nerve cells, by which that pheno- 
menon has been accounted for, and that on a sudden awakening, 
the portions connected with the lowest of the primary forms of 
colour are the first to come into action, the others coming into 
play somewhat later, and almost simultaneously. This would 
completely account for the peculiar crimson colour, and for its 
uniformity of tint over the whole field, excepting the gas flame 
itself, the comparative intensity of whose light may easily be sup- 
posed to have simultaneously aroused all the three sensations in the 
small portion of the retina on which it fell, though it is just pos- 
sible that it also may have appeared crimson for an exceedingly 
short period. I am not aware of any experiments or observations 
having been made with reference to the subject of this note, 
and I hope to have no further opportunities of making them, at 
least in the way in which these were made, but the point is a 
curious one, and worthy of the careful attention of all who may be 
forced to consider it. Professor Clerk-Maxwell informs me that 
he and others have observed that the lowest of the three colour sen- 
sations is the first to evanesce with faintness of light, and that it 
has been asserted to be the most sluggish in responding to the 
sudden appearance of light. This, however, is not necessarily anta- 
gonistic to my explanation, but will rather, if my explanation be 
correct, tend to show a greater interval between the awakening of 
the red, and that of the other colour sensations than that above 
hinted at. 
3. On the Operator £>(v). By Professor Tait. 
(Abstract.) 
By combining, as above, Hamilton’s linear and vector-function 
with his celebrated vector square- root of the negative of Laplace’s 
operator, an operator of great use in physical applications of mathe- 
matics is obtained. With the notation employed in the author’s 
paper “ On Green's and other Allied Theorems,” Trans. B..S.E. 
