TABLE 1

Title of investigation,—Electric-Visual (Red Light).
Experimented on,—Green Frog No. 4.
Harvard Psychological Laboratory,—9.40 A.M., Feb. 28, 1902.
Chronoscope control average, 189σ,—Electric stimulus, 1 Cell.

NO LIGHT.
Number of Experiment.	Reaction-time.
1	152σ
2	145
3	221
4	327
5	263
6	271
7	329
8	215
9	225
10	216
LIGHT BEFORE ELECTRIC STIM.
11	No reaction.
12	No reaction.
13	No reaction.
14	No reaction.
15	No reaction.
NO LIGHT.
16	216

The inhibitory influence of light depends upon the intensity of the electric stimulus. Even a very strong light will not cause much retardation of reaction to a three or four cell current. As the strength of the electric stimulus decreases the delay of reaction increases, until finally there is complete inhibition. At this point, an electric stimulus, to which the frog would react almost invariably when there is no disturbing condition, will fail to cause reaction in the presence of a sudden increase in light intensity.

Merzbacher [150] states that the leg reflex of a frog, so placed that its legs hang free in the air, is greater in response to a given cutaneous stimulus in darkness than in daylight.[151]

Electric and visual stimuli (moving object). For the purpose of determining the effect upon reaction-time to an electric stimulus of stimulation of the eye by a rapidly moving object, experiments were made in which, as in the case of electric and photic stimuli, reactions to electric stimulus alone and to the visual and electric were observed alternately. Thus in the case of each pair of reactions it was possible to note whether the visual stimulus shortened or lengthened the reaction-time. The visual stimulus was given by quickly moving a finger before a window in the reaction-box.

Two series of twenty pairs of reactions each were taken with each of two frogs. In the first series the finger was suddenly moved across the window and the electric stimulus was given either simultaneously or a small fraction of a second later. It was impossible to arrange for accurate measurement of the temporal relations of the two stimuli in the case of these tests. In the second series the finger was moved back and forth before the opening in the reaction-box for an interval of at least a second before the electric stimulus was given.

These experiments, which were in the nature of preliminary tests, yielded the following results. When the stimuli were given almost simultaneously the visual reënforced the electric as was indicated by a shortening of the reaction-time. As appears in the upper part of Table 2, the average time of forty reactions, twenty for each frog, to the electric stimulus was 148^σ,[152] and to the same stimulus when it followed the visual 128^σ. Furthermore, examination of the several pairs of reactions shows, as is indicated in the table, that there were twenty-seven cases in which the visual stimulus caused shortening of the reaction-time (reënforcement of the electric stimulus) to thirteen in which it caused lengthening (inhibition). When the visual stimulus preceded the electric by at least a second, the reaction-time to the electric stimulus was greatly lengthened. The averages are 150^σ for the electric stimulus alone, 178^σ when it was preceded by the visual. In this series there are twenty-five cases of inhibition to fourteen of reënforcement.

Electric and visual stimuli (moving red disc). The indications of the importance of the temporal relations of stimuli, so far as reaction-time results are concerned, furnished by these crude preliminary observations led to a more accurate study of the subject. A revolving disc, which moved at the rate of one revolution per minute, was so arranged that at a certain point it closed an electric circuit in which a magnet had been placed. This magnet attracted a steel arm at the end of which a disc of red cardboard 12 mm. in diameter was suspended. With the making of the circuit the steel arm was drawn downward suddenly and the red disc, by reason of the vibrations of the arm moved rapidly back and forth in front of a window in the reaction-box. In this way the moving object was exposed to view about ten cm. to the right and three cm. in front of the right eye of the frog. The revolving disc, a fraction of a second later, completed the electric stimulus circuit. Thus both stimuli were given automatically, at such an interval apart as the experimenter desired. In the two series of results now to be described the intervals were 0.1 and 0.5 second respectively.