SENSATION AND PERCEPTION. 133 



immersed. Repeat the experiment at intervals of three minutes with the 

 stronger liquids in order, carefully washing the foot after each excitation, by 

 dipping it into a beaker of water. Measure the time which intervenes 

 between the beginning of the excitation and the muscular response in each 

 case, with the aid of a metronome. 



4. Observe carefully the attitude of a brainless frog when left to itself, and 

 its behaviour when placed on its back, on an inclined surface, or in water, as 

 well as when excited by cutaneous stimuli, comparing the phenomena observed 

 with those which exhibit themselves in the spinal cord preparation. 



5. Proceed as in I, substituting a preparation in which, after destruction 

 of the brain, a couple of drops of a o'l per cent, solution of sulphate of strychnia 

 have been injected under the skin of the back. Observe that instead of 

 co-ordinate muscular responses, cutaneous excitation produces under the influ- 

 ence of strychnia, paroxysms of convulsion, in which the body and limbs 

 assume a characteristic attitude. 



V. Sensation and Perception. 



1. Time Occupied in the Simplest Mental Processes 



(see p. 104). To measure the time required for responding to a signal (re- 

 action time or personal time), the simplest plan is to arrange a battery circuit 

 in such a way that it is closed by the same act by which the observer makes 

 the signal, and that it is opened by the response of the observed per- 

 son. Whatever be the nature of the signal, the requirements are: (i) Two 

 Grove's cells arranged in circuit; (2) a break key (a lever resembling in shape 

 a pianoforte key, which when touched breaks a mercurial contact) j (3) a 

 du Bois' key ; (4) an electro-magnet with a light lever attached to its arma- 

 ture ; (5) a chronograph ; (6) a recording surface, of which the rate of motion 

 is not less than I foot per second. The battery, two keys, electro-magnet 

 and chronograph, are arranged in circuit, and in such positions that the electro- 

 magnet lever may be in the neighbourhood of the observed person, and the 

 du Bois' key, cylinder and chronograph, in reach of the observer. On 

 closing the circuit, the lever is drawn towards the magnet and gives the signal. 

 The signal may be an induction shock through the tip of the tongue (in 

 which case an induction coil must be in circuit in addition to the instruments 

 above-mentioned), a touch on the hand given by the lever, a sound or a visible 

 signal, such as a white disk, letter or number, suddenly brought into view. 



2. Tactile and Muscular Sensation. in all the following 



experiments two persons must take part : one of whom must vary the condi- 

 tions without the knowledge of the other, and note the results. In the experi- 

 ments relating to the sensations of pressure, locality, and muscular exertion, 

 the observed person must have his eyes shut. 



The appreciation of Temperature must be tested by immersing the 

 same surface successively in water of slightly different temperatures. The 

 smallest differences can be detected when the temperatures of the liquids com- 

 pared approximate 30 C. 



To test the sensation of Pressure, the hand or other part to be investi- 



