132 Mr. S. Bidwell. On the Recurrent [June 7, 



The experiments were carried out by permission of Professor 

 Clifton, under the Clarendon Laboratory, at Oxford. 

 The result is for 



G, the Newtonian constant of gravitation. . . . 6'6576 X 10~* 

 A, the mean density of the earth 5 '52 70. 



II. " On the Recurrent Images following Visual Impressions." 

 By SHELFORD BIDWELL, M.A., LL.B., F.R.S. Received 



March 27, 1894. 



The earliest recorded observation which I have been able to find 

 of a certain curious phenomenon associated with optical after-images 

 is that of Professor C. A. Young, who published a note on the subject 

 in the year 1872, and proposed that the phenomena should be called 

 "recurrent vision."* He noticed that when a powerful Leyden jar 

 discharge took place in a darkened room, any conspicuous object was 

 seen twice at least, with an interval of a little less than a quarter of a 

 second ; often it was seen a third time and sometimes even a fourth. 

 He thought that the phenomenon suggested the idea of a reflection 

 of the nervous impulse at the nerve extremities, as if the intense 

 impression upon the retina, after being the first time propagated to 

 the brain, was 7'eflected back to the retina and thence again to the 

 brain, thus renewing the sensation of vision. 



A few months later an account of two experiments on the same 

 subject was published by Mr. A. S. Davis. t In the first, a piece of 

 charcoal, one end of which was red-hot, was waved about so as to 

 describe an ellipse or circle a few inches in diameter. A blue image 

 of the burning end was seen following the charcoal at a short distance 

 behind it, the space between the charcoal and its image being ab- 

 solutely dark. The interval of time after which the sensation of 

 blue light succeeded the primary sensation was estimated to be about 

 a fifth of a second. The other experiment was made with a piece of 

 apparatus resembling a photographic instantaneous shutter. The 

 shutter was interposed between the observer's eye and the sky and 

 was covered with pieces of coloured glass, through which momentary 

 flashes of light were allowed to pass. It was found that each flash 

 was, after a short interval, generally succeeded by a recurrent image, 

 the colour of which was quite different from that of the glass. The 

 results of Mr. Davis's observations are summarised below. 



Mr. Davis remarks that except as regards the red glass, the re- 

 current colour does not differ much from the complementary colour, 



* ' Phil. Mag ,' vol. 43 (1872), p. 343. 

 t Ibid., vol. 44 (1872), p. 526. 



