ON LIGHTHOUSE CHARACTERISTICS. 405 



preceding table, that the quarter-second flashes are 

 too short to be perceived with the same certainty 

 as flashes of five or six seconds' duration. Experi- 

 ment alone can answer decisively the question 

 whether, with equal maximum brilliancy in each 

 flash, a flash of quarter-second duration recurring 

 every two seconds, or one of half-second recurring 

 every four seconds, or one of one second recurring 

 every eight seconds, is the most easily to be seen 

 at a great distance or in hazy weather. From 

 physiological experiments already made, it has been 

 concluded that one-tenth of a second is a long 

 enough time to fully excite the sensibility and 

 perceptive power of the eye, and it seems probable 

 that rapidity of recurrence of the contrasts between 

 light and darkness will give a positive advantage to 

 the quicker flash in respect to perceptibility, even 

 when the observer knows in what direction to look 

 for the light ; and when he does not know exactly 

 in what direction to look, which is the practical case 

 of a sailor at sea trying to pick up a light, shortness 

 of the time of invisibility is of supreme importance. 

 All things considered, it seems most probable that 



