(1) The effect of these reagents is not only to increase or diminish the height of the response-curve, but also to modify the time relations. By the action of some the latent period is diminished, others produce a prolongation of the period of recovery. Some curious effects produced by the change of time relations have been noticed in the account given of diphasic variation (see p. 113). (2) The effect produced by a chemical reagent depends to some extent on the previous condition of the wire. (3) A certain time is required for the full development of the effect. With some reagents the full effect takes place almost instantaneously, while with others the effect takes place slowly. Again the effect may with time reach a maximum, after which there may be a slight decline. (4) The after-effects of the reagents may be transitory or persistent; that is to say, in some cases the removal of the reagent causes the responses to revert to the normal, while in others the effect persists even after the removal of all traces of the reagent. #Opposite effects of large and small doses.#--There remains a very curious phenomenon, known not only to students of physiological response but also known in medical practice, namely that of the opposite effects produced by the same reagent when given in large or in small doses. Here, too, we have the same phenomena reproduced in an extraordinary manner in inorganic response. The same reagent which becomes a 'poison' in large quantities 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.