attending sudden Changes of Illumination. 375 



When, however, a visiting card which had been blackened over its 

 whole surface was placed behind the rotating disk, it merely turned a 

 lighter black, or rather grey, in which it was impossible to imagine 

 the slightest tinge of blue. 



A small piece of white paper which was subsequently attached to 

 the middle of the card became blue around its edges when the disk 

 was turned, but the blue did not encroach at all (or if at all, only to 

 a very small extent) upon the black ground. 



When these observations have been made it becomes possible to 

 recognise that the apparently blue lines in the top are themselves 

 really grey, and only bordered externally with blue. 



Experiment VIII. 



The natural conclusion from the observations described above is 

 that if a black disk were suddenly formed upon a bright ground, the 

 disk would for a moment appear to be surrounded by a blue border. 

 I was not successful in devising a satisfactory arrangement for 

 suddenly creating a black disk, but the effect is sufficiently shown in 

 the following manner. 



An aperture 1J in. (3 cm.) in diameter was cut in one side of a 

 wooden box and was covered with white paper ; one half of the 

 aperture could be suddenly covered by a sliding metal shutter which 

 was actuated by a spring : a lamp was placed inside the box. When 

 the shutter was operated, a blue band 1 or 2 mm. wide appeared on 

 the bright ground just beyond and adjoining the edge of the shutter 

 when at rest. Its duration was thought to be slightly longer than 

 that of the red border of other experiments, and it appearently dis- 

 appeared by retreating into the black edge of the shutter. 



When the shutter was moved by hand across the field at a slower 

 speed, its edge was seen to be preceded by a thin blue border, which, 

 when the shutter reached its limiting stop, appeared to reverse the 

 direction of its motion and return into the shutter. 



The blue border is much less conspicuous and more difficult of 

 observation than the red one. In order to see it plainly careful 

 adjustment of the light is necessary. An examination of the effect 

 through coloured glasses was attended by uncertain results. 



Remarks on the Experiments. 



The phenomenon which in the account of Experiment I has been 

 spoken of as a blue halo may be due either to a momentary sympathetic 

 excitement of the nerve fibres of the retina in the neighbourhood of 

 those directly acted upon by the light, or, as I think, less probably, 

 to light scattered by the imperfectly transparent media of the eye. 

 In the latter case its rapid disappearance might be accounted for 



VOL. LX. 2 P 



