234 bulletin: museum of comparative zoology. 



varies fur different stimuli and for different observers. The individual 

 variability is so important in astronomical obsefvatious that it is neces- 

 sary to consider the 'personal equation' of different astronomers in 

 comparing their observations. 



Many observations have been made of the physiological time in the 

 case of different stimuli. Kries und Auerbach ('77), as the result of 

 eight series of investigations on the human subject, give the following 

 figures aa most probably correct : for optical stimuli, 0.193 sec, for acous- 

 tic stimuli, 0.120 sec, for touch stimuli, 0.117 sec. It will be noticed 

 that the simplest reaction-time can, under the most favorable conditions, 

 scarcely be I'educed to 0.1 sec, while it rarely rises much above 0.2 sec 

 The reaction-time for acoustic and touch sensations is nearly the same, 

 but that for optical sensations is nearly twice as great. 



It has been argued that the apparent differences in the reaction-times 

 of the different senses is due to difference in the intensity of tlie stimuli 

 applied, fur an increase in the strength of the stimulus does decrease the 

 reaction-time in any one of the senses ; but according to Ladd ('97) : 

 " There seems good reason to suppose that the reaction-time of sight is 

 necessarily longer than that of hearing or touch, on account of the 

 '^hoto-cliemical nature of its more immediate stimulus. One observer 

 (von AVitlich) has even gone so far as to conjecture that the speed of 

 conduction in the optic nerve is less than that of the other nerves of 

 sense; it is rather to be concluded, however, that the latent time of the 

 sensory end apparatus, and of the cerebral processes by which sensory 

 impulses pass over into motor impulses, is different." 



The deliberate method of experiment used in determining the physio- 

 logical time with reference to optical stimuli, is not of the sort to call 

 into action the short-circuit optic reflex aj>paratus. As there is no element 

 of surprise or danger, the stimulus is probably not intense enough to 

 bring about the shorter reaction, as would be the case in dodging or 

 guarding, in boxing or fencing. The fact that such deliberate reactions 

 to optical stimuli take so much longer than similar reactions to auditory 

 or touch stimuli, we may conjecture to be due to the fact that tlie 

 usual conduction path (^'. e. the chain of neurons involved) for such 

 reactions to optical stimuli is less highly perfected than the similar 

 conduction paths for acoustic and touch stimuli. The reason for this 

 may be that there has been no necessity for such perfection, because 

 from the earliest phylogenetic time there has been this short-circuit 

 optic reflex apparatus to be relied upon in cases of emergency. This, it 

 is true, is merely a speculative suggestion as to a correlation of causes. 



