100 Proceedings of the Royal Society 
The curves of March 4 represent an experiment in which the 
poles of the magnet were reversed, i.e., what was before heated is 
now kept at the temperature of the room ; and these curves show 
what a very great permanent loss was produced by the reversion, 
and also how much greater the rate of diminution of the mag- 
netism was than had been in previous experiments. In after ex- 
periments, such as that of March 6, the permanent loss is not 
nearly so great, and there is also a remarkable decrease in the rate 
of diminution. 
The curves of March 11 represent an experiment in which the 
poles were again reversed ; and, from the approach of the branches, 
we see that the magnet, as it were, becomes accustomed also to 
this treatment, and evinces great unwillingness in losing its mag- 
netism permanently. In this day’s experiment there is a great 
dissimilarity between Curves I. and II. We may look for the 
explanation of this in the alteration of zeros, which possibly may 
be much greater in the one pole than in the other. This supposi- 
tion is strengthened on looking at Curve III., where it will be 
seen the two branches are very widely separated. 
Throughout these experiments we have to acknowledge the 
assistance which W. Campbell, Esq., has rendered us. 
4. On the Physiological Action of Light. No. I. By James 
Dewar, Esq., and John G. M'Kendrick, M.D., of the Uni- 
versity of Edinburgh. 
The authors of this communication have more especially directed 
their attention to the problem of the specific effect produced on the 
retina and optic nerve by the action of light. Numerous hypotheses 
have been made from time to time by physicists and physiologists ; 
but up to the present date, our knowledge of the subject is without 
any experimental foundation. For example, Newton, Melloni, and 
Seebeck, stated that the action of light on the retina consisted of 
a communication of mere vibrations ; Young conjectured that it 
was a minute intermittent motion of some portion of the optic 
nerve; Du Bois-Reymond attributed it to an electrical effect; 
Draper supposed that it depended on a heating effect of the 
