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prism is turned round ; pheenomena very different from those of the 

 crystalline plates he at first examined. These experiments, followed 

 by the more extensive researches of Biot, Fresnel, Brewster, and 

 many more recent investigators, have created a beautiful and import- 

 ant branch of physical optics, designated that of chromatic polar- 

 ization. In this memoir it was also first announced, that while the 

 light from a clouded sky undergoes no modification, that reflected 

 from the atmosphere when the sky is unclouded is polarized, the 

 intensity of the polarization varying with the hour of the day, and 

 the position of the point with respect to the sun. 



In 1816 Arago published a fact which has been generally received 

 as an experimentum crucis between the two rival theories of light, 

 that of emission and that of undulations. Dr. Young had shown 

 that two rays of light, emanating from the same source, act con- 

 jointly when they pass over equal paths, or paths differing by an 

 even number of times a certain very small quantity, and that they 

 destroy each other when this difference is an uneven number of 

 times the same quantity ; whence it follows that the interference of 

 two rays under the stated circumstances produces a series of fringes 

 alternately dark and bright. Dr. Young further ascertained that 

 the interposition of an opaque screen in the path of one of the rays 

 completely prevents the formation of the fringes. Arago ascertained 

 that if one of the rays be made to pass through a thin transparent 

 film, such as glass, the fringes are displaced towards the side to 

 which it is applied, the magnitude of the displacement depending on 

 the thickness of the film. This proves that the transmitted ray has 

 been retarded in its passage, a result conformable with the theory 

 of undulations, but in direct opposition to that of emission. Arago 

 subsequently showed how this principle might be employed to mea- 

 sure the minutest differences in the refractive densities of bodies, to 

 resolve various delicate questions in physics, and in the construction 

 of new meteorological instruments. 



In 1819 Arago and Fresnel published their joint experiments on 

 the action which rays of polarized light exert on each other. By a 

 variety of ingenious methods, they proved that rays polarized in the 

 same plane mutually interfere with each other, producing fringes as 

 in the case of ordinary light ; while rays polarized in planes at right 

 angles manifest no appreciable action on each other. They also 

 showed that two rays primitively polarized at right angles to each 

 other and afterwards brought to similar planes of polarization, pro- 

 duce fringes only when they have proceeded from a pencil originally 

 polarized in the same plane. These new properties enabled Fresnel 

 to give a complete explanation of the production of colours in cry- 

 stalline plates, which Dr. Young had before referred to the interfe- 

 rence of the transmitted rays, though he was unable to explain in 

 what circumstances the interference took place, or why we see no 

 colours unless polarized hght be transmitted through the crystalline 

 plates. 



Hitherto Arago's original researches had been confined to subjects 

 connected with Astronomy and Physical Optics, but the great dis- 

 covery of CErsted gave his inventive ingenuit}^ another direction. 



