RE A CT1ON-TIME, 



963 



Let it be supposed that the change in temperature is sufficient to cause 

 sensation, e.g. of "warmth." The difference between and 6^ expresses the 

 intensity of the excitation. But 1 will not be reached instantaneously. 

 Before it is reached, the end-organs will have possessed a number of different 

 intermediate temperatures t, t lt t.,, etc. If t :i be the thermal state of liminal 

 difference, that is, just sufficient to evoke sensation, it may perhaps be regarded 

 as that which is just sufficient to be a stimulus at the end-organ. On that sup- 

 position, all the time passed between the production of the thermal state t and 

 the thermal state 3 is lost, not in nervous sensifacient action, but in mere applica- 

 tion of the stimulus. The reaction- time, in the ordinary acceptation of the term, 

 commences at the production of t y This period preliminary to the initiation 

 of the sensorial reaction can be reduced by the use of strong thermal stimuli. 



The reaction-time differs according as the source of the thermal 

 stimulus comes into actual contact with the skin or not. With strong 

 " intangible " stimuli, the following periods have been found : l 



The stimulus used for cold was volatilisation of methyl chloride ; that 

 for warmth, the heat from a candle. The reaction-times for the tangible 

 thermal stimuli were found to be shorter very much shorter. The 

 stimuli used were metal discs, cooled or heated. 



Tanzi, "Sulle sensazioni del freddo e del caldo," Milano-Torino,1886. 



