292 REPORTS ON THE STATE OF SCIENCE, ETC. 



The experiments also show, though at present less thoroughly, where and 

 how inhibitions, in the strict and proper sense, are likely to be set up, and 

 how they can be dispelled and lead to sudden anomalous results. 



It still looks as if, given control of all of the determinants indicated above, 

 and very likely of some others which we have not yet investigated in detail, 

 human judgments, or perceptions, of equality or of difference in the case 

 of sensory reactions set up by stimuli of constant physical magnitude do 

 remain remarkably constant also. But it is certain that these constancies 

 of reaction cannot be stated in terms of general laws which refer only 

 to the physical magnitudes compared. Given, for instance, differential 

 adaptation between the two eyes, it is the case that two simultaneously 

 presented visual fields, one of which is 500 times as intense as the other — in 

 terms of physical magnitude — may regularly and constantly be equated. 

 But that amount of physical difference of intensities will, under other 

 circumstances, produce reactions which vary widely from one another. 

 It seems as if, provided the leading groups of physiological and psychological 

 determining conditions are stabilised, human sensory reactions can be 

 equated and differentiated (at least so far as liminal differences are concerned) 

 with a high degree of constancy, and further that under these conditions 

 the equations and differentiations remain relatively remarkably constant 

 for constant values of physical magnitude. We conclude that probably 

 within the human sensory reaction itself there is, or there are, some quality 

 or qualities which enable remarkably consistent comparisons to be made in 

 terms of equality and liminal difference. But what that quality is or those 

 qualities are we cannot at present say. 



Factors Affecting Sensory Thresholds. 

 ByMr.K.y. W.Craik. 



A considerable amount of work has been done in Cambridge, during the 

 last four years, on the influence of various factors on the absolute and 

 differential thresholds of the visual, auditory and tactile senses. It has 

 had two purposes — the discovery of the various conditions which must be 

 controlled, in perceptual experiments, in order that reliable results may be 

 obtained, and the investigation of the mechanism of sensory processes, by 

 finding the variables on which they depend. 



These factors are of various kinds. First, there are such as the subject's 

 state of health, the amount of sleep he has had, the criteria used, the number 

 of readings taken, and the amount of practice allowed. These may be 

 considered as extrinsic and non-specific influences, though they introduce 

 possible sources of error and misinterpretation, if insufficiently controlled. 



Secondly, there are factors such as inhibition, the organisation or gestalt 

 of the field presented, mutual interaction between two similar or dissimilar 

 sense organs, time error, incentives, and knowledge of results, which 

 presumably originate centrally or at some high level. In some cases these 

 have been investigated on their own account,- in the hope that their mode 

 of operation might be discovered ; in other cases, they have been kept 

 constant, so far as possible, in order that the elementary responses of the 

 sense organs to simple forms of stimulation might be studied. 



Thirdly, there are factors which affect either the stimulus presented or 

 the sensitivity of the action of the sense organ itself. The spatial accom- 

 paniments and temporal precursors of the stimulus, and its rate of applica- 

 tion, fall under this category ; they may have direct influence on the stimulus, 

 or on the state of adaptation, and therefore on the sensitivity, of the sense 



