356 



Dr. J. W. Capsiick. 



The excitation of the retina caused by the stimulus of the yellow- 

 ish-green light of the lime-light spectrum, reflected from " white " 

 cardboard in 1/72 sec. (i.e., 160/360 of 1/32) lasts undimiuished for 

 5/288 sec. (i.e., 200/360 of 1/32), i.e., about 1/58 sec. 



Taking the next point G (180) above this last point, we find that 

 the " last " of the stimulus of the same yellowish-green, applied for 

 1/64 sec. is 1/64 sec. Thus, tabulating the results for a few more 

 points including the above two, 



Stimulus for Y.G. applied 1/72 sec. lasts 5/288 ; & 288/5 x 72/1 = 4147 



1/64 1/64 64x64 =4096 

 1/58 1/72 58x72 =4176 

 1/49 1/92 49x92 =4508 

 1/40 1/120 40x120 =4800 

 1/32 1/160 32x160=5120 



Hence the duration of the impression on the retina undiminished 

 appears to decrease as the time of stimulation increases, though 

 within narrow limits of variation one of these quantities is nearly 

 inversely proportional to the other. 



With regard to the total duration of a luminous impression, the 

 writer would point out that nothing has been said in this paper ; the 

 few experiments he has made to measure this, lead to the belief that 

 it is almost of a different order of magnitude from the time during 

 which an impression remains undiminished, and is to be measured 

 by whole minutes rather than by small fractions of a second. 



0n the Kathode Fall of Potential in Gases." By J. W. 

 CAPSTICK, M.A., D.Sc., Fellow of Trinity College, Cam- 

 bridge. Communicated by Professor J. J. THOMSON, F.B.S. 

 Received May 17, Read May 26, 1898. 



It has been shown by Hittorf* that when an electric current passes 

 through a tube containing a gas at a pressure of a few millimetres, 

 there is a rapid fall of potential near each of the electrodes, with a 

 much more gentle fall in the space between, and whilst the fall near 

 the anode and in the positive column varies with the density of the 

 gas and the current strength, the fall near the kathode is constant. 

 Warburgf has made careful experiments on the kathode fall, and has 

 fully established its constancy. If the gas is pure and dry, the 

 electrodes clean, and of a metal not acted on chemically by the gas, 



* ' Wied. Ann./ vol. 20, p. 705. 



f ' Wied. Ann.,' vol. 31, p. 545 ; vol. 40, p. 1. 



