LUMINOUS IMPRESSIONS ON THE EYE. 599 



to that of the uninterrupted light, according as the apparatus was illuminated by 

 sun-light or by gas-hght. Therefore, since it has been shewn that this was not the 

 case, it is evident, that the sun-light and gas-light produced similar portions of 

 their complete impressions on the eye with the same rapidity. 



It has thus been proved, that, when light acts on the eye for short intervals 

 of time, the rapidity of the development of its impression is independent of its ac- 

 tual brightness ; and it seems highly probable that this law extends to the whole 

 time required for the complete production of luminous impressions. For, when 

 it has been found, that lights of very different intensity acting on the eye during 

 5oth of a second, aU produce impressions, having almost exactly jth of the ab- 

 solute brightness of the lights, it seems natm-al to conclude, that they will also 

 produce their complete effect on the eye in exactly equal times. 



I hoped to have been able in this paper to exhibit the results of some experi- 

 ments upon the intensity of impressions of short duration, repeated by different 

 individuals, in order to ascertain whether the rapidity of the production of visual 

 impressions varies much in different eyes. I have only obtained one comparison 

 of this kind, through the kindness of Mr Alexandee Wallace, of the Royal Ob- 

 servatory, Edinburgh, who observed the impression produced on his eye by a disc 

 with a sector of 7 30', revolving 20 times in a second. The following result is 

 the mean of three trials, 



rfj = 4-39; rf, = 42-5; 6 = 0-01067. 



The result of my own experiments gives Z>=0-0137 : which agrees very well 

 vnth Mr Wallace's observations. I trust to be able to obtain some more com- 

 parisons of this kind, in order to ascertain whether the agreement between Mr 

 Wallace's result and my own is to be regarded as proving that visual impressions 

 in the eyes of different individuals, are propagated with nearly equal rapidity. 



VII. On the time which Light requires to produce a fall impression on the Eye. 



I have found, by means of a disc revolving once in a second (see p. 594), that 

 impressions produced by a light acting on the eye for ith of a second, have very 

 nearly the same brightness as the light seen by continuous vision ; but that when 

 light acts on the eye for a shorter time, its apparent brightness is sensibly di- 

 minished. As the brightness of the impression produced by light increases by in- 

 sensible degrees, until, at length, it attains its full intensity, it is obviously al- 

 most impossible, by direct observation, to assign the exact instant when this 

 takes place. The experiments in Sections V. and VI. have proved, that, up to ith 

 of a second, the brightness of a luminous impression is strictly proportional to the 

 time during which light has acted on the eye, and also, that the impression produced 

 in _i_th of a second, has almost exactly ith of the brightness of a full impres- 



