1897.1 on Some Curiosities of Vision. 355 



only do so with the greatest difficulty ; sometimes, indeed, not at all, 

 unless with the assistance of some specially contrived artifice. 



I propose to-night to discuss a few of the less familiar vagaries of 

 the visual organs, and will do my best to assist in the illustration of 

 them. But it will be my part merely to provide the apparatus for 

 the experiments ; the experiments themselves must be carried out by 

 each of you individually. Some of them will, I am afraid, be found 

 rather difficult ; success will depend mainly upon your power of 

 laying aside habit and prejudice and giving close attention to your 

 visual sensations. I hardly dare to hope that every one present will 

 observe all the peculiarities and defects which it is intended to 

 demonstrate, but in case of failure I generally find that there is a 

 comfortable tendency to attribute it not to any deficiency in the 

 observer's power of concentrating his attention, but to the fact that 

 his eyes are not as other mens', and are free from the particular defect 

 which it is desired to bring into prominence. Of course any one is 

 welcome to such an entirely satisfactory opinion. 



Among the most annoying of the eccentricities which characterise 

 the sense of vision is that known as the persistence of impressions. 

 The sensation of sight which is produced by an illuminated object does 

 not cease at the moment when the exciting cause is removed or changed 

 in position, but continues for a period which is generally said to be 

 about ^Q second, but may sometimes be much more or less. It is for 

 this reason that we cannot see the details of anything which is in 

 rapid motion, but only an indistinct blur, resulting from the confu- 

 sion of successive impressions. When I turn this disc, which is 

 painted in black and white sectors, you soon lose sight of the 

 divisions, and if the speed is high enough the w^hole surface appears 

 to be of a uniformly grey hue. If we illuminate the rotating disc 

 by a properly timed series of electric flashes, it looks as if it were at 

 rest, and in spite of the intermittent nature of the light, the black and 

 white sectors are seen quite continuously, though as a matter of fact 

 the intervals of darkness are very much longer than those of 

 illumination. 



The persistent impressions which we have been discussing are 

 often spoken of as positive after-images. 



There is one very remarkable phenomenon accompanying the 

 formation of positive after-images, especially those following brief 

 illumination, which seems, until comparatively recent times, to have 

 entirely escaped the notice of the most acute observers. It was first 

 observed accidentally by Prof. C. A. Young, when he was experiment- 

 ing with a large electrical machine which had been newly acquired 

 for his laboratory. He noticed that when a powerful Leydeu jar 

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

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

 second, the first time vividly, the second time faintly. Often it was 

 seen a third time, and sometimes, but only with great difficulty, even a 

 fourth time. He gave to this phenomenon the name of recurrent 



