098 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 
on the eye in equal times ; from which it obviously follows, that the brightness of 
an impression on the eye increases nith a rapidity exactly proportional to the bright- 
ness of the light which produces it. 
This conclusion seemed so remarkable, that I determined to try whether the 
direct light of the sun produced a given portion of its impression on the eye with 
no greater rapidity than ordinary artificial light. For this purpose I made use of 
a selaometer, represented in fig. 5, where K L represents a plate of brass with 
two apertures A B, 4th of an inch in diameter, and half an inch distant. A plate 
of ground glass is placed before the apertures, and behind the aperture B, a 
tube B C is fixed, in which is placed a Nicow’s polarizing prism. A longer tube 
BD, is fitted so as to turn freely upon the outside of the tube C D, and another 
Nicow’s prism is placed in its further extremity, so that, by turning round the 
tube B D, the illumination of the aperture B can be varied at pleasure. A disc 
EF, with a sector of 7° 30 revolves rapidly in front of the plate, by means of the 
band HI passing over the pulley G, so as to project beyond the aperture A, which 
is only visible when the sector passes before it at each revolution of the disc.* The 
apertures were first illuminated by gas-light, and the disc being made to revolve 
so rapidly as to produce a continuous impression, the apparent brightness of the 
apertures was made equal by turning one of the prisms. When the apparatus 
was next illuminated by the direct light of the sun at noon, and the disc made 
to revolve so as to produce a uniform impression, the apertures were still equally 
bright, although the position of the prisms remained unaltered. This experiment 
was repeated several times with the same result, and a similar result was ob- 
tained when moon-light was compared with gas-light. Now the effect of turning 
round the prism is to diminish the brightness both of the sun-light and gas-light 
in the same proportion. Since, therefore, the two apertures were always equally 
bright, it follows, that the apparent brightness of the aperture behind the revolv- 
ing disc, had also, in both cases, the same ratio to that of the light seen by unin- 
terrupted vision. But the ratio of the apparent brightness of the aperture behind the 
revolving disc to that of the direct light, evidently depends on the rapidity with 
which the light acts on the eye at each passage of the sector before the luminous 
aperture. Hence it is obvious, that if the sun-light and gas-light required dif- 
ferent times to produce like portions of their total effect on the eye, the apparent 
brightness of the fiashes produced by the revolving disc would have different ratios 
* By means of this arrangement, the brightness of the impressions produced during the revolu- 
tion of the disc, can be compared with the light transmitted through the aperture B. Since the in- 
tensity of a ray of polarized light when transmitted through a doubly-refracting crystal, varies as the 
square of the cosine of the inclination of the principal section of the crystal to the plane of polariza- 
tion of the ray ; by attaching an index to the tube B D, so as to measure the angle through which it 
has been turned, the intensity of the transmitted light might be estimated, and thus the brightness 
of the impressions produced by the revolving disc might be determined. (See Supplément au Traité 
de la Lumiére de Sir J. F. W. Herscner. Par A. QuerExer, p. 595.) 

