Mr. A. S. Davis on Recurrent Vision, 527 



It appeared to me probable^ from the experiment with the burn- 

 ing charcoal, that there would be such a difference of colour ; 

 and the experiments I am about to describe prove that there is. 



Not having a powerful electrical machine at hand, I con- 

 trived the following apparatus for producing an instantaneous 

 illumination, and thus exhibiting the phenomenon of recurrent 

 vision. 



In a board about S feet long and a foot wide, at about a foot from 

 one end of it a rectangular hole was cut 3 inches in the direction 

 of the length of the board and 5 inches in the direction of its 

 breadth. Upon this board another, smaller board was made to 

 slide and act as a shutter to the hole. In this shutter a hole 

 was made similar to the hole in the large board, so that when the 

 shutter was partially drawn up the two holes coincided. A strong 

 elastic band was attached to the board and the shutter in such a 

 manner that, when the shutter was raised, the band acted upon 

 it to pull it down, in the same way as that in which a bowstring 

 acts upon an arrow. A thick rug was nailed round the edges of 

 the board ; and when this was thrown over the head, a dark space 

 was formed which could be momentarily illuminated by drawing 

 up the shutter and letting it spring back. 



The following experiments were made :— - 



I. The hole in the board being turned towards the objects in 

 a room and the shutter being drawn up and let go, a recurrent 

 image of the objects was seen; but the illumination was in 

 general too feeble and the impression too momentary for the eye 

 to judge of the colours of the objects. When, however, a bright 

 coloured object was placed in a strong light, the colour of the 

 recurrent image was seen to be different from the actual colour 

 of the object. By gaslight or feeble daylight the recurrent 

 image appeared twice if the object was white, or nearly white. 

 The recurrent colour of a white object is of a blue tint. 



II. Various coloured glasses were placed before the aperture^ 

 and the board was turned towards the sky. 



With a deep-blue glass the recurrent image was a greenish 

 yellow. With a green glass and with a yellow glass it was a 

 reddish blue. With a single red glass, which gave an orange- 

 red light, it was a red-blue. 



With two red glasses superposed, which produced a pure red 

 light, no recurrent image was seen, however bright the hght. 



In the case of the blue, green, and yellow glasses the effect 

 w^as much better seen when the intensity of the light was mode- 

 rated by placing against the hole, along with the glass, one or 

 two sheets of white paper ; the recurrent image was stronger 

 compared with the primary image, and the interval of compara- 

 tive darkness between the two images was more clearly perceived, 



