SECTIONAL TRANSACTIONS.— I. 603 



Dr. F. W, Edridge-Green, C.B.K— Simultaneous Colour Contrast. 



1. The colours seen by simultaneous contrast, are duo to the exaggerated perception 

 of a real, objective, relative difference which exists in the light reflected from the two 

 adjacent surfaces. 



2. A certain difference of wave-length is necessary before simultaneous contrast 

 produces any effect. This varies with different colours. 



3. A change of intensity of the light of one colour may make evident a difference 

 which is not perceptible when both colours are of the same luminosity. 



4. Simultaneous contrast may cause the appearance of a colour which is not 

 perceptible without comparison. 



5. Both colours may be affected by simultaneous contrast, each colour appearing 

 as if moved further from the other in the spectral range. 



6. Only one colour may be affected by simultaneous contrast, as when a colour 

 of low saturation is compared with white. 



7. When a false estimation of the saturation or hue of a colour has been made the 

 contrast colour is considered in relation to this false estimation. That is to say, 

 the missing (or added) colour is deducted from (or added to) both. 



8. A complementary contrast colour does not appear in the absence of objective 

 light of that colour. 



9. The negative after-images of contrasted colours are complementary to the 

 colours seen. 



Afternoon. 



Joint Meeting with Sections D (q.v.) and K for communications on 

 Experimental Biology. 



Monday, September 10. 



Dr. R. H. Thouless. — Resistance and Polarisation in the Human Skin. 



When the electrical resistance between two non-polarisable electrodes placed on 

 different parts of the skin is measured by a rapidly alternating current (of several 

 thousand cycles per sec), the resistance is found to" be very much less than when it 

 is measured by a direct current. Einthoven regards this as due to the fact that the 

 major part of the resistance to a direct current is due to the high resistance of a thin 

 la,yer of the skin which behaves as the dielectric to a condenser system of sufficiently 

 high capacity to offer only slight resistance to a rapidly alternating current. Thus 

 the main part of the resistance measured by such a current is not that of the skin 

 but of the underlying tissues. Gildemeister, on the other hand, attributes the main 

 part of the apparent resistance to a direct current, not to the skin resistance (which on 

 this theory is small), but to a back E.M.F. of polarisation, the effect of which is very 

 largely ehniinated by the use of rapidly alternating currents. These may be called 

 the ' capacity ' and the ' polarisation ' theories respectively. Failure to obtain the 

 psycho-galvanic reflex (P.G.R.) with such alternating currents is explained on the 

 capacity theory by the supposition that the reflex is a change in skin resistance alone 

 and not in that of the underlying tissues ; on the polarisation theory by the 

 supposition that the reflex is not a change in resistance but in polarisation. 



Gildemeister's principal evidence for the polarisation theory was obtained from an 

 experiment in which the body was balanced on a Wheatstone bridge with a variable 

 inductance (L) in the fourth arm adjusted to bring the bridge to a sharp balance by 

 compensating for the phase change in the alternating wave due to the capacity (or 

 polarisation) of the system. He showed that if AW were the difference between the 

 resistance as measured on the bridge and the value to which it approximated as the 

 frequency was increased indefinitely, then, on the capacity theory, AW/L should be 

 constant for all values of the frequency (/). He found that this was not the case but 

 that AW//L was more nearly constant (as would be expected on the polarisation 

 theory). 



Repetition of this experiment confirms Gildemeister's result. Experiments reported 

 to this section last year which pointed to the capacity theory must, therefore, be 

 regarded as valueless. Einthoven's own experiments are also inconclusive. 



On the polarisation theory, we should expect an increase of direct current to be 

 accompanied by a decrease of apparent resistance, since as saturation of polarisation 



