a Miscellaneous Intelligence. 443 
V. Miscettaneous In TELLIGENCE. 
* 
; On the Gradual Production of Luminous inpresiiinci on the Eye, 
nd other erage of Vision; by Wiut1am Swan, F.R.S.E. (Proc. 
Roy. Soc. Edinb., 1849, ii, 230. )—The object of this communication 
was to toca the relation between the apparent brightness of a light, 
he time during which it acts on the eye. In order to examine the 
intensity of luminous impressions of short duration, the author made use 
f dises, having sectors of known angles cut out of their circumferences, 
which were made to revolve at known velocities between the eye and 
aperture in it is visible at each revolution of the disc throughout the 
apertures are illuminated by gas flames behind them, 
iS ectangular 
n of glass is placed half way. between the apertures, with its faces 
ed at angles of 45° té the line joinipg their mea so that they 
Seen in apparent contact by reflecti from the faces of the prism, 
their relative brightness can thus be compared wit ca great nicety. 
es other light from its sc When the disc is put in the 
parent brightness of ee pm behind it is instantly diminished 
the equality of the apparent brightness of the apertures i the 
ens is restored, by increasing the distance of the light from the 
he ratio of the brightness of the impression produced 
‘i light —s the revolution of the disc, to the brightness of its 
Pression, when seen by uninterrupted vision, is that of the squares of 
: Pianos of the other light from the aperture in its screen. 
The copied are the principal results obtained by means of this 
‘appara 
(1) ‘When the eye receives, from a light of common intensity, a 
‘succession of flashes of ~~ duration, which succeed each other so 
Een as to produce a uniform impression, this impression will also 
é a constant aint, ceeded s the number = flashes in a given 
time se inversely with the duration of each 
(2. nT e brightness ia the impression Sokeand by flashes of light of 
a given neni, — h succeed each other so rapidly as to produce a 
n the eye, is proportional to the number of flashes 
in a given re 
* 
