516 



the results of his observations with the formulae of M. Cauchy ; 

 and the accordance of the calculated and observed numbers is 

 striking in the highest degree. 



Another highly important memoir of M. Jamin, read before the 

 Academy of Sciences, and published in the Annales de Chimie ' for 

 1850, relates to the reflexion of light at the surface of transparent 

 media. According to the formulae of Fresnel, when light polarized 

 perpendicularly to the plane of incidence is incident on such a medium, 

 and the angle of incidence is increased from to 90, at a certain 

 angle (the polarizing angle) the reflected light vanishes, and in 

 passing through this angle the reflected vibration changes sign, or in 

 other words, the phase is changed by half an undulation. It had 

 already been recognized, that in the case of very highly refracting 

 substances these results differ very sensibly from the phenomena 

 actually observed ; and by observations on Newton's rings when 

 formed between a glass lens and a plate of diamond, Mr. Airy long 

 ago showed that in the case of diamond the light does not wholly 

 vanish at the polarizing angle, and that in passing through this 

 angle, on increasing the angle of incidence, a continuous but rapid 

 retardation of phase takes place, amounting to a quantity sensibly 

 equal to half an undulation, while the angle of incidence is increased 

 a few degrees. But except in the case of substances of very high 

 refractive power, it had been supposed that Fresnel' s formulae sen- 

 sibly represented the phenomena observed. Such was the state of the 

 subject when M. Jamin commenced a series of most elaborate expe- 

 rimental researches, which resulted in showing that the phenomena 

 above mentioned, instead of being confined to a few of the most 

 highly refractive bodies, were of almost universal occurrence, though 

 for most bodies the only sensible changes of phase were crowded 

 within a small range of incidence, and the quantity of light reflected 

 at the minimum was very small. A very striking result of these 

 researches was, that the change of phase had not always the same 

 sign. It appeared that for bodies having a higher refractive index 

 than about 1*46, the change was of the same sign as in the case of 

 metals, while for bodies having a refractive index much lower it 

 was the opposite. Thus the case of no reflexion at a particular 

 angle, accompanied by an abrupt change of phase amounting to half 

 an undulation, or, as in this special instance it may be more simply 



