LUMINOUS IMPRESSIONS ON THE EYE. 585 



ceives light at each revolution of the disc maybe varied, by altering the diameter of 

 the luminous circle, or its distance from the eye, or from the disc, or by changing 

 the distance of the pencil of transmitted rays from the centre of the disc. But 

 in the experiments to be afterwards described, these elements remained constant, 

 and the effect was modified only by altering the angles of the sectors, or the rate 

 of revolution of the discs. It may, however, be proper to observe that, as the time 

 during which each element of the surface remains visible is independent of the 

 magnitude of the luminous object ; so also, as might be anticipated, the apparent 

 brightness of the surface is independent of the ratio which the portion visible at 

 once through the sector bears to the whole area. For, in repeating the same ex- 

 periments with circular luminous objects of different diameters, while the angle 

 of the sector and the velocity of the disc were constant, it was found that the 

 apparent brightness of the luminous circle was not sensibly affected by varying 

 its diameter. 



In order to compare the brightness of the impressions produced by light 

 seen thi-ough the sectors of revolving discs, in the manner now described, 

 with its brightness when seen by uninterrupted vision, the following arrangement 

 was devised, which, for the sake of convenient reference, may be termed a 

 selaometer (from fffXas, brightness), to indicate its use in measuring the brightness 

 of luminous impressions. This apparatus, represented in Fig. 2, is supported on 

 a stout plank A B. AC and B D are screens with circular apertures C, D, an inch 

 in diameter, to which are fitted pieces of ground glass, cut fi'om the same plate, 

 in order to secure similarity of surface. The apertures C, D, are illuminated by 

 the gas-burners, L, S, * which are supplied by flexible tubes, so that their dis- 

 tances from the screens can be varied at pleasure by sliding their supports M, M, 

 along the plank A B, in a groove cut in it for that purpose. An axis E H, carry- 

 ing the disc I K, revolves between conical points in supports, one of which E F is 

 seen in the figm-e. This axis is put in motion by a band passing over the pulley 

 Q, and over a wheel driven by means of a winch ; and it is made of sufficient 

 length to admit of a second disc revolving in front of the screen AC simultaneously 

 with the disc I K. The brightness of the apertures in the screens is observed by 

 means of a rectangular prism of glass N 0, placed half-way between them, with 

 its faces inclined at angles of 45° to the line CD joining their centres. By this 

 means the light passing through the apertures, and dispersed by the ground- 

 glass, is reflected fi-om the faces of the prism to the eye at P, and the images of 

 the apertures being seen in apparent contact, as represented at C, D', their rela- 

 tive brightness can be compared with great nicety.f The driving-wheel is made 



* The gas-light used in all the experiments described in this paper, was that of coal-gas 

 burned by a No. 2 swallow-tail jet. It will be seen that the numerical results, afterwards obtained, 

 do not depend on the absolute brightness of this light. 



t Screens covered with black paper, which are not represented in the figure, were used to pro- 

 tect the eyes from the action of extraneous light, and also to intercept any rays, whose influence might 

 have otherwise affected the accuracy of the experiments. 



VOL. XVI. PART V. 7 N 



