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, notAvithstand- 

 ing its sensible dm-ation, 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 jglg^ to i of a second. 



4. The intensity of the impression produced by light which acts on the eye for 

 -jlg of a second is almost exactly ith 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 ith 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 igooosth of what its apparent brightness would become if its 

 duration were prolonged to ygth of a second ; and the brightness of electric light 

 increases with the tension of the electricity. 



