LUMINOUS IMPRESSIONS ON THE EYE. 



597 



In fig. 3, the line a e contains the projections of these observations, which are 

 denoted by crosses ; and it nearly coincides with the line a b, containing the projec- 

 tions of the observations in the last Table, shewing that the ratio of the brightness 

 of an impression of given duration to that of the absolute brightness of the light 

 which produces it, is almost exactly the same in both sets of experiments. On 

 thus comparing the apparent intensities of the flashes exhibited in the above Table 

 with the similar results in the preceding one, it will be seen that although the ab- 

 solute intensity of the light is only half as great as formerly, the time required for 

 the propagation of the luminous impression on the eye remains unaltered ; while 

 both sets of experiments prove that the brightness of a luminous impression caused 

 by a light of given intensity is proportional to the time during which the light 

 acts on the eye. 



On repeating the experiment with the sector of ith of the circumference, re- 

 volving once in a second, with the fixed light 24 inches from the screen, the mean 

 of ten trials gave 



rfi = 22-27 ; d.-^ = 48-46; b = 0-2112. 



The ratio of the apparent brightness of the flashes to that of the light seen 

 by continuous vision is, in this case, almost exactly the same as in the preceding 

 experiments, as will be seen from the following comparative view : — 



The conclusion to be derived from these results will be distinctly apprehended 

 by reference to fig. 3, where the ordinates e/, cf, and df, represent the ap- 

 parent intensities of the lights shewn in the above table. In order to pre- 

 vent misunderstanding, it is necessary to observe, that although the absolute 

 brightness of the lights used in the three experiments given in the table are in 

 the ratio of the numbers 1, 2, and 16, they are all represented in the figure by 

 the same line a g ; and since the lines ef, cf, df, are nearly equal, they may be 

 regarded as having the same ratio to a g, the slight differences between them ob- 

 viously resulting from errors of observation. It thus appears that after an inter- 

 val of ^gth of a second, the three lights of very different intensity have all pro- 

 duced the same portion of their total effect on the eye ; the impression in each case 

 having nearly ith of the absolute brightness of the light. 



Lights of different intensity, therefore, produce like portions of their total effect 



VOL. XVI. PAET V. 7 Q 



