212 



Mr. Gr. J. Burch. On the Belation of 



not merely the light of the larger disc, but in addition the light of 

 the smaller disc. Yet in spite of this, with certain relations of in- 

 tensity and duration between the two images, the total sensation 

 evoked by the larger quantity of light was less, so that the small disc 

 appeared as a black spot on the larger disc. I was curious to ascer- 

 tain to what extent this principle could be carried. By the following- 

 experiment it may be demonstrated in a striking manner. A card- 

 board disc, A, figs. 3, 4, 200 mm. diameter, of which 180° is black and 



Tig. 3. Fig. 4. 



A 



the rest white, has a slit about half a millimetre wide cut at the- 

 junction of black with white. It is made to revolve so that black 

 precedes and white follows the slit. An incandescent lamp B is- 

 placed behind the disc, and another C in front of it, so as to throw a 

 strong light upon its surface. While the disc is moving slowly the 

 incandescent filament of the lamp B, seen through the slit as it passes 

 across, looks bright against the white card, but when a certain speed is- 

 reached it appears as a black thread against a brighter background. 



In spite of the very short duration of the intense light of the fila- 

 ment, the fatigue induced by it is out of all proportion to the sensa- 

 tion it excites, and in consequence the less fatiguing illumination of 

 the white card produces a greater effect on the senses than the sum of 

 the sensations due to the filament and the subsequent light. With a 

 single flash the filament looks, to the rested eye, black all over, but 

 with a succession of flashes there is generally an appearance as though 

 the luminous filament were partly covered by an opaque black thread, 

 but could be seen in places behind it. This I think is due to the 

 shifting of the images on the retina, which after the first flash is no- 

 longer in a uniform condition. The retina is, as it were, scarred with 

 after-images, and the ratio of illumination to length of flash which suits 

 one part is incorrect for another. 



When a red glass is placed over the lamp the reversed image of the 

 filament is green if the card is not quite white, or if the light falling 

 on it is yellowish, but blue-green or blue if it is illuminated by sun- 

 light or the arc lamp. The use of coloured glasses, however, so far 

 diminishes the light that in most cases a disc with a different flash 

 ratio has to be employed, and the lamp C placed at a greater 

 distance. 



