LUMINOUS IMPRESSIONS ON THE EYE. 585 
ceives light at each revolution of the disc may be 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 through 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 «Aas, brightness), to indicate its use in measuring the brightness 
of luminous impressions. This apparatus, represented in Fig. 2, is supported on 
a stout plank AB. AC and BD are screens with circular apertures C, D, an inch 
in diameter, to which are fitted pieces of ground glass, cut from 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 EH, carry- 
ing the disc I K, revolves between conical points in supports, one of which EF is 
seen in the figure. 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 IK. The brightness of the apertures in the screens is observed by 
means of a rectangular prism of glass NO, 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 from 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.;+ 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, 
+ 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. 7N 
