THE ARCHIPELAGO OF BREHAT. 133 



to steer their course with safety. Where the light 

 alternately appears and disappears in the manner we 

 have described, the apparatus for lighting is said to 

 be on the principle of a revolving light. 



Borda's mode of lighting can, unfortunately, only 

 be applied to lighthouses of this kind ; and is in- 

 applicable for buildings which are illumined by a 

 Jixed light, that is to say, where the light must be 

 visible from every point of the horizon at one and 

 the same time. This method of illumination must, 

 however, necessarily be adopted, since it is impos- 

 sible to vary the lights and obscurations in such a 

 manner as to impart to each lighthouse sufficient indi- 

 viduality of character to distinguish it from all others. 

 Much, therefore, remained to be done. There had 

 existed in France for many years a lighthouse- 

 commission, the members of which, occupied with a 

 hundred other duties, had scarcely done anything 

 towards the solution of this problem, when M. Arago* 

 proposed to undertake a series of experiments, 

 on condition that he might receive as his coadjutors 

 in the work MM. Mathieu and Frestielf, whose ad- 



* [A short biographical sketch of Arago is given in the Ap- 

 pendix, Note X.] 



f Fresnel, who was a Member of the Institute, ranks amongst the 

 most illustrious cultivators of physical science. He especially 

 devoted himself to the study of the most delicate phenomena of 

 light. " Among the great observers, who have preceded him," says 

 M. Pouillet, " we can mention no one who has exhibited more 

 inventive power in his experiments, more precision in his measure- 

 ments, or greater depth of thought in his deductions." It is to 

 Fresnel that we are indebted for that solid basis on which the now 

 generally accepted undulatory theory rests, which consists in regarding 

 K 3 



