528 Mr. A. S. Davis on Recurrent Vuion. 



wlien this v/as done. With the red glass, on the other hand, the 

 recurrent image was only seen when the light was strong. 



III. When, instead of producing a momentary illumination, 

 the shutter was raised so that the two holes coincided, and after 

 being held for a short time was let go, the image of the hole 

 became for an instant before disappearing of the same colour as 

 its recurrent image; but in this case there was no interval of 

 darkness before the change of colour took place. We may con- 

 clude from this experiment that the induced excitation, which, 

 when the illumination is momentary, gives rise to a recurrent 

 image, lasts as long as the light which produces it, beginning 

 and ending a fifth of a second after it; but being much feebler, 

 it is only seen after the primary light has disappeared. 



IV. The shutter being removed, a large piece of black card- 

 board with a hole in the middle of it, about half an inch in dia- 

 meter, v/as moved quickly about before the hole in the board. 

 A recurrent image of the hole followed the primary image in the 

 same way as in the experiment with the burning cliarcoal. When 

 the coloured glasses were placed before the hole, the colours of 

 the recurrent images were the same as in the previous experi- 

 ments \\Ai\i the same glasses. 



The complementary colours of the coloured glasses were ascer- 

 tained by fixing small pieces of white cardboard against them 

 and holding them up to the light. The white cardboard took 

 by contrast the complementary colour of the glass against which 

 it was fixed. In this way it was found that the complementary 

 colour of the blue glass was yellow, of the green glass a blue- 

 red, of the yellow a blue, and of the red a blue-green. It thus 

 appears that, with the exception of the red glass, the recurrent 

 colour does not difi'er much from the complementar}^ colour. 



The recurrent image given by white light is, as I have already 

 remarked, of a blue tinge. It follows that the less saturated 

 any colour is the bluer will be its recurrent colour; for a colour 

 which is not saturated may be regarded as a mixture of white 

 light and a saturated colour. This explains the fact that, though 

 the recurrent colour of a deep-blue glass is a greenish yellow, yet 

 the recurrent colour of a blue object sufficiently light-tinted to 

 give a recurrent image is of a blue tinge. In fact all light-co- 

 loured objects give a recurrent image of a more or less blue 

 tinge ; for they all differ but little from white, 



A recurrent image of an object may also be produced without 

 any apparatus whatever. To do this, place the right hand over 

 the eyes so that the palm of the hand covers the right eye and 

 the fingers the left eye. If the middle finger be then raised 

 for a moment so as to admit light for as short a time as possible 

 into the eye, a recurrent image of any light-coloured object held 



