INORGANIC RESPONSE 149 
the destructive breaking-down of the tissue, followed 
by its renovation. Some support was at first given 
to this chemical theory by the bleaching action of light 
on the visual purple present in the retina, but it has 
been found that the presence or absence of visual 
purple could not be essential to vision, and that its 
function, when present, is of only secondary importance. 
For it is well known that in the most sensitive portion 
of the human retina, the fovea centralis, the visual purple 
is wanting; it is also found to be completely absent 
from the retine of many animals possessing keen 
sight. 
(2) Electrical theory.—The second, or electrical, theory 
supposes that the visual impulse is the concomitant of 
an electrical impulse; that an electrical current is 
generated in the retina under the incidence of light, 
and that this is transmitted to the brain by the optic 
nerve. There is much to be said in favour of this view, 
for it is an undoubted fact, that light gives rise to 
retinal currents, and that, conversely, an electrical 
current suitably applied causes the sensation of light. 
Retinal currents.—Holmegren, Dewar, McKendrick, 
Kuhne, Steiner, and others have shown that illumination 
produces electric variation in a freshly excised eye. 
About this general fact of the electrical response there 
is a widespread agreement, but there is some differ- 
ence of opinion as regards the sign of this response im- 
mediately on the application, cessation, and during the 
continuance of light. These slight discrepancies may 
be partly due to the unsatisfactory nomenclature—as 
regards use of terms positive and negative—hitherto in 
