192 FRESNEL. 



of it, you can not see it by looking up through the upper 

 surface; but if you bring the finger down below the level 

 of the water, and on the farther side of it, then, with a lit- 

 tle care in placing it right, you will see it reflected in the 

 upper surface seen from below that is, from the under side 

 of the upper surface. 



These explanations will give the reader a general idea 

 of the fundamental principles of Fresnel's invention for 

 managing the light in light-houses, and will enable any 

 one, when he visits a light-house constructed on these prin- 

 ciples, to understand what he sees, when, without this pre- 

 liminary knowledge, the complicated combination of rings 

 of glass, and mirrors, and prisms would form for him only 

 an intricate and bewildering maze. 



Indeed, the number and the variety of the modes in 

 which these general principles are applied, and the vast 

 extension which the system has received since Fresnel first 

 inti'oduced it, and w r hich is necessary to produce the great 

 variety of luminous effects required for distinguishing the 

 different lights from each other, are such that it is the 

 work of a lifetime to understand the whole subject in all 

 its details. 



Fresnel was a highly-educated man and a profound math- 

 ematician, and he made his discoveries, not by any lucky 

 accident, but by the most careful and thorough study of 

 the philosophy of optics. He was educated as an engineer 

 in the military schools of France, and was subsequently 

 appointed to important posts under the French govern- 

 ment first in respect to bridges and roads, and afterward 

 in relation to the establishment and management of light- 

 houses on the coasts. It was from the profound investi- 

 gations that he made in connection with his official duties 

 that his discoveries and inventions resulted. 



And yet, notwithstanding the great eminence as a math- 



