28 PRACTICAL PHYSIOLOGY 



E. TIME TAKEN IN VISUAL PERCEPTION 



(The method of driving the recording drum, of varying its speed 

 and of altering its direction, of fixing and of smoking the glazed 

 paper should be demonstrated.) 



METHOD. Connect with the bench terminals marked F, or with 

 a dry cell in one circuit, an electro-magnetic time-marker C, and 

 with two mercury keys, A and B, separated by a considerable 

 length of wire, so that when both keys are closed the current 

 passes and the lever on the marker is depressed. Bring the 

 points of the lever of the marker C lightly against the smoked 

 surface of a rapidly revolving drum. Key B being open, one 

 student stands beside the drum and lever watching this lever, and 

 holding the handle of the closed mercury key, A, which he must 

 open the moment he sees the lever depressed. Another student 

 now closes the key, J5, in such a way that the subject can neither 

 see nor hear the closing. The lever is thus suddenly depressed. It 

 is released again when the first student opens A, when the point of 

 the lever will spring up. Below the record thus obtained, a time 

 tracing in ^ sec. is made. To do this, set the drum going, and, 

 when it is revolving uniformly, strike a tuning fork vibrating one 

 hundred times per second on the thigh so as to set it vibrating and 

 holding it by the handle, and with the two limbs in the vertical plane, 

 bring the wire attached to one limb against the drum under the 

 trace and run off a time trace. Each tooth on the tracing represents 

 T ^ eT second. 



The interval between the application of the stimulus and the 

 resulting action is measured and recorded. At least two obser- 

 vations must be made, and unless the time measurements correspond 

 closely a third must be carried out. 



After writing with a pin or other instrument name, date, and 

 nature of experiment, the tracing is fixed by passing it through 

 varnish and hanging it up to dry. Preserve the trace and measure- 

 ments in your note-book. 



A similar method may be used for measuring the time taken in 

 perceptions with the other senses. 



Lesson V. To be provided for each pair of Students. 



1. Discs of paper of pure spectral colours. 2. Sheets of white paper. 

 3. Black square on white paper, and white square of same size on black paper. 

 4. Red square on white paper, and similar red square on green paper. 

 5. Electro -magnetic time-marker. 6. Recording drum covered with smoked 

 paper. 7. Two mercury keys. 8. Lengths of insulated copper wire of about 

 6 feet. 9. A tuning fork vibrating l-100th second with marker on one limb. 



To be provided for the Class. 

 Perimeter* Stereoscopes with some suitable figures. 



