me 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 retinae 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.#--Holmgren, 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 difference of opinion
as regards the sign of this response immediately 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 vogue and
partly also to the differing states of the excised eyes observed.

Waller, in his excellent and detailed work on the retinal currents of
the frog, has shown how the sign of response is reversed in the moribund
condition of the eye.

As to the confusion arising from our present terminology, we must
remember that the term _positive_ or _negative_ is used with regard to a
current of reference--the so-called current of injury.

[Illustration: FIG. 95. RETINAL RESPONSE TO LIGHT
The current of response is from the nerve to the retina.]

When the two galvanometric contacts are made, one with the cut end of
the nerve, and the other on the uninjured cornea, a current of injury is
found which _in the eye_ is from the nerve to the retina. In the normal
freshly excised eye, the current of response due to the action of light
on the retina is always from the nerve, which is n