186 



FRESNEL. 



to confine myself to the strict boundaries of truth, and 

 the consciousness which I have of never having trans- 

 gressed them, it should happen that this eloge should be 

 accused of some exaggeration. Though I must avow it 

 would be a reproach for which I should feel little as the 

 friend of Fresnel, if it were incumbent on me to repel 

 it, it would be solely in the capacity of the organ of the 

 Academy : the office which I this day fill, in the name 

 of my colleagues, ought to be marked by a precision 

 and severity as great as that of the exact sciences with 

 which it is concerned. 



REFRACTION'. 



The labours of Fresnel almost exclusively relate to 

 optics. In order to avoid tedious repetitions, I shall 

 classify them, without regard to the order of dates, in 

 such a way as to collect in a single group all those which 

 relate to analogous subjects. The first which will engage 

 my attention are the phenomena of refraction. 



A straight rod partly immersed in water appears bent 

 or broken ; the rays by which we see the part immersed 

 must, therefore, have changed their route or have been 

 broken themselves, in passing out of the water into the 

 air. It was till lately supposed that to this one observa- 

 tion we were to restrict the entire knowledge of the an- 

 cients on the subject of refraction. But in exhuming 

 from the dust of libraries, where so many treasures are 

 yet concealed, a manuscript of the optics of Ptolemy, it 

 has been found that the School of Alexandria had not 

 confined itself to establishing the mere fact of refraction ; 

 for this work includes from all incidences, numerical de- 

 terminations, tolerably exact, of the deviations of the 

 rays, whether they pass out of air into water or into 



