SOME CURIOSITIES OB 1 VISION. 



199 



and sometimes, but only with very great difficulty, even a fourth time. 

 He gave to this phenomenon the name of recurrent vision; it may 

 perhaps be more appropriately denominated the Young effect. 



We have here a machine XDresented to the institution by Mr. Wiins- 

 hurst, which is a giant in comparison with that used by Professor 

 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 one-fifth of a 

 second it will make another brief appearance. Some of you may per- 

 haps 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 dis- ^^- *" 



charge. 



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 

 succession 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 phe- 

 nomenon 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 defined, and though its apparent lumi- 

 nosity is feeble, it can no doubt be easily seen by most 

 of you. The varied colors 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 indicated 

 by dotted lines.) 



I returned to the subject three or four years ago, with the primary 

 object of 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 attrib- 

 uted 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. 1 



In investigating the influence of color upon the Young effect, two 



Fig. 1. 



1 Proc. Roy. Soc. ; Vol. LVI, p. 132 (1894). 



