356 



Mr. STielford Bidwell 



[March 5, 



vision : it may perhaps be more appropriately denominated the 

 Young effect. 



We have here a machine presented to the Institution by Mr. 

 Wimshurst, which is a giant in comparison with that used by Prof. 

 Young, and I hope by its means to be able to show the effect to every 

 one present who will give a little attention. Look in the direction of 

 some object which is exposed to the light of the discharge : the object 

 will be seen for an instant at the moment when the spark passes and 

 you hear the crack, and after a dark interval of about ^ second it will 

 make another brief appearance. Some of you may perhaps see even 

 a second recurrent image. Under certain conditions I myself have 

 observed no less than six reappearances of an object which was 

 illuminated by a single discharge. 



Twelve years ago I called attention to a very different method of 

 exhibiting a recurrent image. The apparatus used for the purpose 

 _ consists of a vacuum tube mounted in the usual 



way upon a horizontal axis capable of rotation. 

 "When the tube is illuminated by a rapid suc- 

 cession of discharges from an induction coil, and 

 is made to rotate very slowly (at the rate of 

 about one turn in two or three seconds) a very 

 curious phenomenon may be noticed. At a 

 distance of a few degrees behind the tube, and 

 separated from it by a clear interval of darkness, 

 comes a ghost. This ghost is in form an exact 

 reproduction of the tube ; it is very clearly de- 

 hned, and though its apparent luminosity is 

 feeble, it can no doubt be easily seen by most 

 of you. The varied colours of the original are, 

 however, absent, the whole of the phantom tube 

 being of a uniform bluish or violet tint. If the 

 rotation is suddenly stopped, the ghost still 

 moves steadily on until it reaches the luminous 

 tube, with which it coalesces and so disappears. 

 (See Fig. 1, where the recurrent image is indi- 

 cated by dotted lines.) 

 . I returned to the subject three or four years 



ago, with the pnmg^^y ^^y^JQ^.^ ^f ascertaining whether or not the Young 

 effect was identical with one which had recently been discovered by 

 Charpentier, and which will be referred to presently. A certain 

 phenomenon which I had attributed to the Young effect was quoted 

 by Charpentier as exemplifying his own newly-observed one. I 

 found, however, that the two effects, though both of an oscillatory 

 character, were in fact quite distinct from one another. The results 

 of my experiments in relation to this and other allied matters were 

 embodied in a communication to the Royal Society.* 



Fig. 1. 



Proc. Roy. Soc. vol. Ivi. p. 132 (1894). 



