antities may act as a stimulant when applied in small doses.
This is seen in record fig. 94, in which (_a_) gives the normal
responses in water; KHO solution was now added so as to make the
strength three parts in 1,000, and (_b_) shows the consequent
enhancement of response. A further quantity of KHO was added so as to
increase the strength to three parts in 100. This caused a complete
abolition (_c_) of response.

[Illustration: FIG. 94.--OPPOSITE EFFECTS OF SMALL AND LARGE DOSES (TIN)
(_a_) is the normal response; (_b_) is the stimulating action of small
dose of potash (3 parts in 1,000); (_c_) is the abolition of
response with a stronger dose (3 parts in 100).]

It will thus be seen that as in the case of animal tissues and of
plants, so also in metals, the electrical responses are exalted by the
action of stimulants, lowered by depressants, and completely abolished
by certain other reagents. The parallelism will thus be found complete
in every detail between the phenomena of response in the organic and the
inorganic.

CHAPTER XVII

ON THE STIMULUS OF LIGHT AND RETINAL CURRENTS

Visual impulse: (1) chemical theory; (2) electrical theory--Retinal
currents--Normal response positive--Inorganic response under
stimulus of light--Typical experiment on the electrical effect
induced by light.

The effect of the stimulus of light on the retina is perceived in the
brain as a visual sensation. The process by which the ether-wave
disturbance causes this visual impulse is still very obscure. Two
theories may be advanced in explanation.

#(1) Chemical theory.#--According to the first, or chemical, theory, it is
supposed that certain visual substances in the retina are affected by
light, and that vision originates from the metabolic changes produced in
these visual substances. It is also supposed that the metabolic changes
consist of two phases, the upward, constructive, or anabolic phase, and
the downward, destructive, or katabolic phase. Various visual substances
by their anabolic or katabolic changes are supposed to produce the
variations of sensation of light and colour. This theory, as will be
seen, is very complex, and there are certain obstacles in the way of its
acceptance. It is, for instance, difficult to see how this very quick
visual process could be due to a comparatively slow chemical action,
consisting of the destructive breaking-down of the tissue, followed by
its renovation. So