ot directly stimulated
by light, to the retina, that is, from the less excited to the more
excited (fig. 95). This current of response flows, then, in the same
direction as the existing current of reference--the current of
injury--and may therefore be called _positive_. Unfortunately the
current of injury is very often apt to change its sign; it then flows
through the eye from the cornea to the nerve. And now, though the
current of response due to light may remain unchanged in direction,
still, owing to the reversal of the current of reference, it will appear
as _negative_. That is to say, though its absolute direction is the same
as before, its relative direction is altered.

I have already advocated the use of the term _positive_ for currents
which flow towards the stimulated, and _negative_ for those whose flow
is away from the stimulated. If such a convention be adopted, no
confusion can arise, even when, as in the given cases, the currents of
injury undergo a change of direction.

#Normal response positive.#--The normal effect of light on the retina, as
noticed by all the observers already mentioned, is a positive variation,
during exposure to light of not too long duration. Cessation of light is
followed by recovery. On these points there is general agreement amongst
investigators. Deviations are regarded as due to abnormal conditions of
the eye, owing to rough usage, or to the rapid approach of death. For
just as in the dying plant we found occasional reversals from negative
to positive response, so in the dying retina the response may undergo
changes from the normal positive to negative.

The sign of response, as we have already seen in numerous cases, depends
very much on the molecular condition of the sensitive substance, and if
this condition be in any way changed, it is not surprising that the
character of the response should also undergo alteration.

Unlike muscle in this, successive retinal responses exhibit little
change, for, generally speaking, fatigue is very slight, the retina
recovering quickly even under strong light if the exposure be not too
long. In exceptional cases, however, fatigue, or its converse, the
staircase effect, may be observed.

#Inorganic response under the stimulus of light.#--It may now be asked
whether such a complex vital phenomenon as retinal response could have
its counterpart in non-living response. Taking a rod of silver, we may
beat out one end into the form of a hollo