LUMINOUS IMPRESSIONS ON THE EYE. 



595 



1. The fixed light was placed six inches behind the screen before which the 

 disc revolved. The sector had an angle of O'^ 27', or gL of the circumference of 

 the disc ; and the disc revolved once in a second. To equalise the brightness of 

 the apertures in the screens, when both were seen by continuous vision, the light 

 behind the second screen was placed at a distance (d^) of 5-3 inches. But, 

 when the disc revolved, this light had to be withdrawn to a distance (t?,) of 46"3 

 inches. This experiment was repeated ten times, with the following mean 

 results: d^ = 5-ll; d, = 50-29 ; and the brightness of the flashes 6=0-0103, the 

 brightness of the light seen by continuous vision, being unity. 



2. A sector of 0'54', or ^^ of the circumference, was next used, and ten 

 experiments made as before, from which 



d^ = 5-23; ^2= 35-24 and 6=0-022. 



3. With a sector of 1° 48', or ^Jo of the circumference, 



d,=5-19; ^2 = 23-52; 6=0-0487. 



4. With a sector of 3° 36', or ^ j „ of the circumference, 



rf,=4-9; ^2 = 1508; 6 = 0-1056. 



5. With a sector of 7° 12', or j\ of the circumference, 



c?i = 5-13; d^ = U-3; 6=0-2061. 



6. With a sector of 15", or ^\ of the circumference, 



c?i = 5-02; (/2 = 7-64; 6=0-4317. 



With each sector, the mean of ten results was taken ; and at each successive 

 trial, the flame was alternately drawn from the screen, or pulled towards it in 

 equalising the apparent brightness of the apertures in the screens. The following 

 Table contains the results of these experiments, the brightness of the light seen 

 by continuous vision being expressed by unity : — 



The results of these experiments are shewn in fig. 3, where the observed 

 intensities of the light, denoted by small circles, are projected with the corre- 

 sponding times during which it acted on the eye ; and it will be observed, that the 

 line acb, shewing the increase of the apparent brightness of the object, with the 

 time during which it remains visible, is very nearly straight ; which proves that 



