172 Intelligence and Miscellaneous Articles. 



effective intensity) follows a law of increase a little less rapid thar 

 the consumption of oil, as indicated by experiment. 



The second memoir relates to the nocturnal transparence of th< 

 atmosphere. The observations made by the lighthouse-keepers on 

 the visibility of the neighbouring lights consist in notiug, three 

 times each night, whether each of those lights can or cannot be 

 perceived ; so that at the end of a certain number of years it is 

 known how many times per cent, each of the lights is visible. On 

 the other hand, the equation of the luminous ranges, and the graphic 

 Table which I have constructed for its ready solution in all cases, 

 make known, for each of the lights observed, what is the lowest 

 state of transparence at which it ceases to be seen from the place 

 of observation. The information supplied by the keepers makes 

 known, therefore, during what fraction n of the total duration of 

 the nights the transparence of the air remains above a certain 

 value represented by a coefficient a ; and as we have a series of 

 corresponding values of n and a, we can, by constructing the points 

 which have these values for coordinates, determine a more or less 

 regular carve. This curve generally permits us to determine the 

 degree of transparence above w T hich the atmosphere is maintained 

 during a given fraction of the year. It is found, for example, 

 that duriDg half the year the transparence-coefficient remains above 

 O910 per kilometre on the Ocean, and above 0*932 on the Mediter- 

 ranean. Similar curves constructed for different sections of the 

 coast, and for the four seasons of the year, enable us to account for 

 the variations undergone by the transparence of the atmosphere. 



Lastly, in a third memoir, I examine the impressions produced 

 on the visual organ by the scintillating lights obtained by the 

 more or less rapid rotation of a system of lenses with explosions. 

 Experiment shows that, if a luminous discharge is caused to pass 

 before the eye, the impression experienced goes on diminishing in 

 proportion as the velocity is augmented. If a series of discharges, 

 succeeding one another at equal intervals, passes, with low velo- 

 cities each discharge produces nearly the same effect as if it were 

 isolated ; then, when the velocity increases, the impression, while 

 diminishing in intensity, is prolonged until the commencement of 

 that produced by the following burst, so that one has the sensation 

 of a light agitated with a more or less rapid trembling. If the 

 velocity be further augmented, the trembling tends to disappear, 

 and at last there is the sensation of a continuous light which 

 possesses nearly the intensity of that which would be got by spread- 

 ing uniformly- round the horizon the quantity of light contained in 

 the explosions. 



These are the facts of which I have sought the theoretic expla- 1 

 nation. I first admitted that, when a luminous source acting on 

 the eye vanishes suddenly, the impression grows weaker with a 

 velocity at each instant proportional to the value of the impression, 

 according to the law given by Newton for cooling. From this we 

 can easily deduce the value of the impression experienced by the 

 eye w T hen submitted to the action of a light variable with time ; in 



