199 
It seemed desirable to extend the observations to impressions 
formed on the eye in intervals of time still shorter than *001 of a 
second ; and it may seem that this could be accomplished, either by 
diminishing the angle of the sector, or by increasing the diameter 
or velocity of rotation of the disc. There are obviously, however, 
limits to the narrowness of the sector, and to the diameter of such 
discs as can be used conveniently ; and the velocity with which the 
disc may be driven is also limited, for when the number of revolu- 
tions exceeds about ten in a second, the successive impressions, which 
it is proposed to observe separately, become blended into a single 
nearly uniform impression, owing to their persistence on the retina. 
The instrument now described to the Society is devised for the pur- 
pose of separating a single impression out of the multitude of im- 
pressions made by a rapidly revolving disc, so as to render it pos- 
sible to observe the brightness of isolated visual impressions formed 
by light acting on the eye for extremely short intervals of time. 
The instrument consists of a train of wheels and pinions by which 
a disc having a sector cut in it is driven with great velocity. 
The numbers of teeth in the wheels and pinions are so arranged 
that each wheel, as well as the disc, makes ten revolutions for one 
revolution of the wheel by which it is driven. Each of the two last 
wheels of the train, which are of solid metal, has a hole pierced in it, 
through which light transmitted by the sector can pass to the eye ; 
and the wheels are so placed that at each hundredth revolution of the 
sector, and only then, the sector in the disc and the holes in the 
wheels come into the same straight line, so that the eye of the ob- 
server receives a single flash transmitted through the holes in the 
wheels. The result of this arrangement is, that although the disc be 
driven at the rate of a hundred revolutions per second, so that the 
impressions produced by the successive flashes transmitted by it 
when seen by the unassisted eye would be blended into an uniform 
impression, yet the observer, looking through the holes in the wheels, 
receives only a single flash of light once a second. The brightness 
of the observed isolated flashes may be ascertained by photometrical 
means, similar to those employed by the author for the same pur- 
pose in 1849, and which he has fully described. 
An Instrument for producing Isolated Luminous Impressions of 
extremely short duration, varying from one-tenth to one millionth of 
a second, was shown. 
