Yerkes, The Sense of Hearing in Frogs. 289 



I found it desirable, as did Merzbacher, to observe the 

 ■movements of a shadow of the leg on the scale and thus read 

 the amount of movement, rather than to watch the leg itself 

 and attempt to project it upon the scale. 



As is indicated in the figure the auditory and tactual stim- 

 uli were given automatically by means of a swinging pendulum 

 (/*) which was held in position by the magnet a until released 

 by the experimenter. Early in its swing the pendulum turned 

 the key in thus completing a circuit which caused the auditory 

 stimulus to be given; later in the swing the key 71 was turned, 

 and the tactual stimulus thus given through the magnetic release 

 of the lever /. The interval between the auditory and the tact- 

 ual stimuli could be varied from.i" to .9" by change in the 

 position of the key n. For giving the two stimuli simultan- 

 eously a double hand key was employed.^ 



The auditory stimulus was either the sound of a quick 

 hammer blow or the ringing of an electric bell for a certain 

 interval. Figure 4 shows the bell. It was placed 80 cm. from 

 the frog, and in order that the influence of vibrations might be 

 avoided was suspended from the pendulum frame. When the 

 hammer was used it was placed 60 cm. from the frog, on the 

 pendulum table. The frog and the apparatus for tactual stim- 

 ulation occupied a separate table which was not disturbed by 

 the jars of the pendulum table. The tactual stimulus was giv- 

 en by a rubber cone, T, Fig. 4, 2 mm. in diameter at its apex. 

 This rubber point, after the electric release of the lever to which 

 it was attached, struck the frog at the middle point of a line 

 drawn between the posterior margins of the tympana. The 

 intensity of the stimulus could be varied by weighting the lev- 

 er (see w in the figure). 



Under the conditions of experimentation described above, 

 a tactual stimulus regularly causes a reflex movement of the 



^A full account of this method and the results of a study of the phenom- 

 ena of auditory-tactual reinforcement and inhibition may be found in Pfiilger' s 

 Archiv, Bd. 107, S. 207, IQ05, under the title "Bahnung und Hemmung der Reac- 

 tionen auf tactile Reize durch akustiche Reize beim Frosche." In this connec- 

 tion I mention only such aspects of the investigation as bear on the subject of 

 audition. 



