702 Mr. A. B. Wood on the Velocities of the 



light, that we secure the desired result, namely, a sudden 

 brightening and appearance of white sectors on a dark field- 

 As the two ends o£ the diameter of the disk move in opposite- 

 directions, these two cases may be simultaneously observed 

 and compared. 



It is thus seen that the explanation in terms of retinal 

 fatigue fails to account for the facts, and it seems unnecessary 

 to postulate either it or else the nerve " paralysis " suggested 

 by Mr. Mallock as the principal factor at work. The follow- 

 ing explanation seems more in accordance with the facts : — 

 So long as the retina is absolutely at rest, and the white and 

 dark sectors follow one another at intervals short compared 

 with the period of persistence of vision, the disk appears 

 uniformly illuminated. But if the retina is set in motion 

 even for a small fraction of a second, say by a slight mechanical 

 shock, or by the eye involuntarily following the motion of 

 the sectors, and if the direction of this motion is such that 

 the -white sectors remain on any given portion of the retina 

 for a longer interval than they otherwise would, the impression 

 of light over the areas occupied by the dark sectors has time 

 enough to die away appreciably, and we thus get the illusion 

 of stationary white sectors on a dark ground. A movement 

 of the retina in the opposite direction should, however, pro- 

 duce little or no perceptible effect, provided the rotation of 

 the disk is sufficiently rapid. This is exactly what is found 

 in experiment. 



Calcutta, May 10, 1915. 



LXXIV. The Velocities of the a Particles from Thorium 

 Active Deposit. By A. B. Wood, M.Sc, Oliver Lodge 

 Fellow and Assistant Lecturer in Physics, University of 

 T Aver pool* . 



[Plate XVII.] 



RUTHERFORD and Robinson, in the Philosophical 

 Magazine of October 1914, give an account of an 

 extremely careful and accurate determination of the mass 

 and velocity of the a-particles from radium C. Assuming 

 the value of the velocity thus obtained and applying it in 

 Geiger's formula (r 8 = &Il) connecting ranges and velocities 

 of a-particles, a list of the initial velocities of expulsion of 

 a-particles from the various a-ray products is tabulated. 

 More recently, Tunstall and Makowerf have made a direct. 



* Communicated bv the Author. 

 t Phil. Mag. xxix. Feb. 1915. 



