164 O. N. Rood—Nature and Duration of 
neous spark. If, however, the illumination of the ‘spark last 
to the left till superposition has been attained), then, owing to 
4 e retention of impressions on the retina, the 
ES distinction between the black and white lines 
[aes will be exactly obliterated, and a tint of gray 
¢ _ produced, as can be shown by a construction. 
Be In fig. 4 the first and last views only are ee 
B 
but as the action is an unbroken one, we mu 
reverse. nd in general, if we set the number 
of views equal to some odd number greater 
than unity, we shall obtain for the interior por 
acs pt of white, 
tions parts of black, with 
