140 Mr. S. Bidweli. On the Recurrent [June 7, 



This experiment, though not conclusive, is clearly in favour of the 

 probability that the blue and bluish-green recurrent colours apparently 

 observed when the yellow and orange portions of the spectrum are 

 tested separately are due merely to an effect of mental judgment, and 

 not to any cause of a physiological nature. 



There are, therefore, four independent facts which are consistent 

 with the conclusion that luminous recurrent images are due to a 

 reaction of the violet nerve fibres only. 



(a.) With white light the recurrent colour is violet. 



(6.) In the recurrent image of the complete spectrum no colour 

 but violet can be detected. 



(c.) A pure red light, however intense, gives no recurrent image. 

 It is generally supposed by the supporters of the Young- 

 Helmholtz theory that red light has no action upon the 

 violet nerve-fibres. 



(d.) The apparently blue colour of the ghost of simple spectrum 

 yellow is just as well produced by a compound yellow con- 

 sisting of green and red, the latter of which is inert when 

 tested separately. 



The path of the revolving spot of light is generally marked by a 

 phosphorescent track, which, when the rate of revolution is not less 

 than one turn in 1^ sees., often forms a complete circle. The bril- 

 liancy of this luminous trail seems to vary with different observers, 

 in some cases appai'ently being so intense that the recurrent image 

 cannot be distinguished from it at all. The trail is due to the usually 

 feeble continuation of the after-image, of which the bright initial 

 stage constitutes the recurrent image. 



A spot of red light, although it is never followed by a ghost, is 

 always considerably elongated daring its revolution, and its colour 

 ceases to be uniform, the rear portion assuming a light bluish-pink 

 tinge. However small the spot of light is made, and however high 

 the speed of revolution, no complete separation of the spot into red 

 and pink portions has ever been effected. 



In the experiment next to be described the Charpentier effect and 

 the recurrent image are made to exhibit themselves simultaneously, 



Experiment 6. 



Two blackened zinc disks, 15 cm. in diameter, from each of which 

 two opposite quadrants were cut out.* were mounted in contact with 

 each other on a horizontal axis, driven by clockwork and making one 

 turn in 1| sees. By slipping the disks over one another round their 



* It was found necessary to cut out two quadrants instead of only one, in order 

 to balance the disks and secure uniform rotation. 



