QUANTITATIVE ESTIMATES OF SENSORY EVENTS 295 



Following Sherrington, interaction between two similar or dissimilar sense 

 organs has been employed as a diagnostic sign of the level at which any 

 perceptual process was taking place. Irregularity and disinhibition also 

 provide clues, as noted above. Peripheral factors influencing the frequency 

 and intensity range of the ear have also been investigated by electrical 

 recording of auditory nerve-impulses (Hallpike and Rawdon-Smith, 1934, 

 I ; Pumphrey and Rawdon-Smith, 1936 ; Hallpike, Hartridge and Rawdon- 

 Smith, 1937 ; Adrian, Craik and Sturdy, 1938), and localisation of pitch 

 discrimination in the cochlea assisted by the same method (Hallpike and 

 Rawdon-Smith, 1934, 2). 



In the eye, the effects of adaptation are so marked and regular as to give 

 ground for analysing the behaviour of the eye (in regard to brightness 

 discrimination and acuity) into two parts — its behaviour at different test 

 illuminations when maintained in a constant state of adaptation throughout, 

 and its power of adaptation to different illuminations. To borrow an 

 analogy from Lythgoe (1935), the eye behaves rather like an ammeter which 

 can be set to various ranges (adaptation to different intensities) and gives 

 readings over a certain scale of currents when set to any one range (the 

 brightness sensitivity of the eye at various test intensities when adapted to 

 a fixed illumination throughout). It then appears that the eye sets itself 

 automatically to its most efficient range, for any given illumination, if allowed 

 sufficient time to adapt ; for it is found that the differential threshold is 

 lowest when the adapting and test illuminations are equal. 



It has further proved possible, in some work at present in progress, to 

 make monocular comparisons between fields simultaneously exposed to the 

 two eyes, when one eye is bright adapted and the other adapted to some 

 lower illumination or to darkness. Under these conditions, judgment of 

 equality to within a scatter of plus or minus 10 % may be obtained 

 although, owing to the two eyes being differently adapted, the physical 

 intensities may differ five hundred-fold. That so constant a judgment of 

 equality can be made between sensations whose physical stimuli are very 

 widely different indeed, raises once again the problem as to whether sensa- 

 tions may not be in some sense measurable, that is to say whether there may 

 not be some quality of the sensory responses themselves that enables them 

 to be equated, or differentiated in equal appearing steps. It is, at any rate, 

 clear that what value of physical intensity will be constantly equated to 

 another, or just differentiated from another, depends upon many other 

 conditions than the physical intensities themselves. 



References. 



Adrian, E. D. : /. Physiol., 65, 273. 1928. 



Adrian, E. D., Craik, K. J. W., and Sturdy, R. S. : Proc. Roy. Soc. B., 125, 



435- 1938- 

 Craik, K. J. W. : /. Physiol., 92, 406. 1938. 



and Zangwill, O. L. : Brit. J. Psych, (in press). 1938. 



Drew, G. : Brit. J. Psych., 27, 279. 1936. 



Gelb, a., and Granit, A. R. : Zts.f. Psych., 12, i. 1923. 



Grindley, G. C. : Brit. J. Psych., 27, 189. 1936, i. 



Brit. J. Psych., 27, 86. '1936, 2. 



Hallpike, C. S., Hartridge, H., and Rawdon-Smith, A. F. : Proc. Roy. Soc. B., 



122, 175. 1937- 

 and Rawdon-Smith, A. F. : /. Physiol., 81, Pr. 25. 1934, i. 



Nature, 133, 614. 1934, -■ 



Lane, C. E., andWEGEL, R. L. : In H. Fletcher — ' Speech and Hearing,' 176. 



London, Macmillan & Co. 1929. 

 Lythgoe, R. J. : Trans. III. Eng. 1, 3. 1935. 

 and Tansley, K. : Proc. Roy. Soc. B., 105, 73. 1929. 



