605 
of Edinburgh, Session 1871-72. 
in the experiments represented by curves I. and II. was not the 
same as the one used in that represented by curve III. ; the latter 
was a thin, very hard steel magnet, the former thicker and softer, 
and it may be seen from the curves that the hard steel parted with 
its magnetism less readily than the soft. 
From these experiments it follows also that 
dm 
dt ’ 
or the rate of 
change of magnetism with temperature, is not constant for each 
temperature, but depends in some way or other upon the state of 
the magnet. 
When the above experiment was repeated with an electro-magnet 
in the copper pot instead of a permanent magnet, it was found that 
while at a temperature of 500° F. the power of the permanent 
magnet is very much lessened, that of the electro-magnet, provided 
the intensity of the current remain constant, is unaltered. 
2. Note on a Singular Property of the Retina. 
By Professor Tait. 
While suffering some of the annoyances seemingly inseparable 
from re-vaccination at too advanced an age, I was led to the curious 
observation presently to be described. I was unable to sleep, ex- 
cept in u short and far between ” dozes, from which I woke with 
a sudden start, my eyelids opening fully. I found by trial that 
this state of things became somewhat less intolerable when I 
lay on my back, with my head considerably elevated. In this 
position I directly faced a gas jet, burning not very brightly, placed 
close to a whitish wall, and surrounded by a ground glass shade, 
through which the flame could be prominently perceived. The 
portions of the wall surrounding the burner were moderately illu- 
minated, and hyperbolic portions above and below somewhat more 
strongly. I observed, on waking, that the gas flame seemed for 
a second or two to be surrounded by a dark crimson ground, though 
itself apparently unchanged in colour. Gradually, after the lapse 
of, at the very utmost, a couple of seconds, everything resumed its 
normal appearance. As this phenomenon appeared not only to be 
worthy of observation in itself, but to furnish me with something 
definite to reflect upon, which is far the best alleviation of annoy- 
VOL. VII. 4 M 
