LUMINOUS IMPRESSIONS ON THE EYE. 603 
rison of the nearly instantaneous electric spark of high tension, with the apparently 
continuous light of voltaic electricity. For since the latter light, notwithstand- 
ing its sensible duration, does not appear brighter than the former, it must 
obviously be greatly inferior in intrinsic brightness. 
It may now be useful to recapitulate the principal results of the experiments 
described in this paper. 
1. When the eye receives a succession of flashes of equal duration from 
a light of constant intensity, which succeed each other so rapidly as to produce a 
uniform impression, the intensity of this aggregate impression will also be con- 
stant, provided the number of flashes in a given time varies inversely with the 
duration of each. 
2. The brightness of the impression produced by flashes of light of a given 
intensity, which succeed each other so rapidly as to produce a uniform impres- 
sion on the eye, is proportional to the number of flashes in a given time. 
3. When light of a given intensity acts on the eye for a short space of time, 
the brightness of the luminous impression on the retina is exactly proportional to 
the time during which the light continues to act. This law has been proved to 
be true for impressions lasting from ;,4,, to i, of a second. 
4. The intensity of the impression produced by light which acts on the eye for 
169 Of a second is almost exactly th of the apparent brightness of the light 
when seen by uninterrupted vision ; and the time required for light to produce its 
full effect on the eye seems to be about 3th of a second. 
5. Lights of different intensities produce their complete impressions on the 
eye in equal times, so that the light of the sun requires the same time as common 
artificial light to produce its impression on the eye. 
6. The brightness of an impression on the eye increases with a rapidity 
exactly proportional to that of the light by which it is produced. 
7. Rays of different refrangibility act on the eye with equal rapidity. 
8. The apparent brightness of the spark produced by electricity of high ten- 
sion is only about zgdg00th of what its apparent brightness would become if its 

duration were prolonged to 75th of a second; and the brightness of electric light 
increases with the tension of the electricity. 
