434 Sir D. Brewster on Luminous Impressions on the Retina. 
ing the liquids should be a metal; any conducting substance 
will answer the same purpose; nor is it necessary that the 
substance should have a sensible size, for a single atom may 
produce a new arrangement of the molecules of the substance 
with which it is in contact, as in the case of platina with oxy- 
gen and hydrogen, when these substances are mixed, by the 
intensity of the attraction of its atoms, and this arrangement 
will depend on that of the substance causing decomposition ; 
the arrangement of the atoms composing the ordinary mole- 
cules being similar in both. 
It would thus appear that voltaic action is nothing more 
than chemical action taking place in circumstances that en- 
able us to observe many of the phenomena to which it 
gives rise, and which we have no means of observing in ordi- 
nary cases; and that chemical action is the result of the 
tendency of the molecules to arrange themselves in a state of 
pe aera in the same manner as ordinary mechanical 
orces. 
LXXI. On the Combination of prolonged direct luminous Im- 
pressions on the Retina with their complementary Impressions. 
By Sir Daviv Brewster, K.H., D.C.L., F.R.S., and 
V.P.RS. Edin.* 
ie is well known that when we have looked steadily at an 
object for some time, and then shut our eyes, the object 
continues visible, and of its natural colour, or the impression 
of it is prolonged on the retina during the third part of a se- 
cond ; and it is equally well known that if the object is coloured 
and pretty luminous, it will appear in its accidental or comple- 
mentary colours after the third of a second has elapsed. ‘This 
last phenomenon is easily seen, but I have met with many 
persons who have never seen distinctly the first phenomenon, 
unless in experiments such as those exhibited by the thau- 
matrope, and similar pieces of apparatus. 
In making some of these experiments in the morning before 
the eye has had its sensibility diminished by exposure to light, 
I have observed a singular combination of these two phzeno- 
mena, which I believe has not been previously noticed. 
If, when our eyes have been for a few minutes shut, we 
open them and look steadily at the pattern of a carpet, sup- 
pose a red pattern upon a green ground, and then suddenly 
shut our eyes, we shall see the ved pattern upon a pinkish-green 
ground, the red being very deep and approximating to black. 
* Communicated by the Author. 
