302 Prof. A. Anderson and Mr. H, N. Morrison on 



of the iris afforded just such a "physiological process"*. 

 The iris contracts when the eye is exposed to a bright red or 

 to a bright green light. There must therefore be some 

 relative brightness of the two lights which tends equally to 

 close the iris, and this may afford the measure required. 

 The flicker adjustment is complete when the iris has no 

 tendency to alter under the alternating illumination. 



This question was brought home to me very forcibly, 

 when in 1875 I fitted the whole area of the window of a 

 small room with revolving sectors after the manner of 

 Talbot. The intention was to observe, more conveniently 

 than when the eye is at a small hole, the movements of 

 vibrating bodies. The apparatus served this purpose well 

 enough ; but incidentally I was much struck with the re- 

 markably disagreeable and even painful sensations experienced 

 when at the beginning or end of operations the slits were 

 revolving slowdy so as to generate flashes at the rate of perhaps 

 3 or 4 per second. I soon learned in self-defence to keep my 

 eyes closed during this phase ; and I attributed the dis- 

 comfort to a vain attempt on the part of the iris to adjust 

 itself to fluctuating conditions. 



It is clear, I think, that we have here a common element 

 in variously coloured lights, such as might serve as the basis 

 of coloured photometry. I suppose that there would be 

 no particular difficulty in observing the movements of an 

 iris, and I would suggest that experiments be undertaken 

 to ascertain whether in fact the flicker match coincides with 

 quiescence of the iris. Should this prove to be the case, the 

 view suggested would be amply confirmed ; otherwise, it 

 would be necessary to turn to some of the other possibilities 

 discussed by Mr. Ives. 



XXV. On Contact Difference of Potential, and the Action of 

 Ultra-Violet Light. By Prof. A. Anderson, ALA., and 

 H. N. Morrison, B.ScA 



T^HE contact difference of potential of two metals depends 

 on the state of the surfaces of the metals. The effect 

 of making the surface of a metal bright and clean is to render 

 it more electropositive to other metals, and the magnitude of 

 the effect is different for different metals. The following 

 is the result of a careful experiment on this point. The 



* If my memory serves me, I have since read somewhere a similar 

 suggestion, perhaps in Helmholtz. 

 f Communicated by the Authors. 



