122 ANNUAL OF SCIENTIFIC DISCOVERY. 



mercury, and thence to deduce, by the well known law of Brewster, 

 the index of the refraction of these substances, which he found to be 

 two for steel and four for mercury. This method does not give the 

 quantity of polarized light, but the relative proportion between that 

 quantity and the total light. To modify the process so as to render it 

 suitable for that purpose, it is essential to commence by the gradua- 

 tion of the pile of glass plates, in order to ascertain for a given inci- 

 dence what was the quantity of polarized light which the glass plates 

 neutralized. By causing a completely polarized ray to fall on a plate 

 of rock-crystal, perpendicular to its axis, in such a manner that the 

 principal section of the crystal coincides with the primitive plane of 

 polarization, we have, on turning the plate, two images, varying in 

 intensity according to the law of the square of the cosine, but which, 

 from the thinness of the plate, entirely superpose on each other. These 

 two images are polarized in two perpendicular planes, and form neutral 

 light, by their superposition, as long as the quantity of light is equal 

 in the two ; but if this quantity happens to vary in either one of the 

 images, the other, having all its light neutralized, will leave a certain 

 residue of polarized light, which the pile of glass plates will entirely 

 obliterate. The green color observed in light, when it is obliged to 

 traverse a greater thickness of glass than ten plates, has prevented M. 

 Arago, up to the present time, from extending the limits of the table 

 for representing the quantities of polarized light ; but they hope, that 

 by using very transparent glass, having oxide of zinc for its basis, 

 which they are at present occupied in preparing, that they shall be 

 able to extend the table considerably. This process of polarimetry will 

 allow of our measuring the quantity of light reflected and refracted at 

 great angles, which the usual phometetric method was always found, 

 by M. Arago, incapable of effecting. 



ON THE HYPOTHESES RELATING TO THE LUMINOUS .ffiTHER, AND AN EX- 

 PERIMENT DEMONSTRATING THAT THE MOTION OP BODIES ALTERS THE 

 TELOCITY WITH WHICH LIGHT PROPAGATES ITSELF IN THEIR INTERIOR. 



THE following important communication was read to the French 

 Academy, by M. Fizeau : 



Many hypotheses have been proposed to account for the phenomena 

 of aberration in accordance with the undulatory theory ; but from the 

 want of any definite ideas as to the properties of the luminous asther, 

 and its relations to ponderable matter, none of them can be considered 

 as strictly proved. These various hypotheses may be reduced to three 

 principal ones. They refer to the state in which the asther existing in 

 transparent bodies may be considered to be. 



1st. This aether is either adherent, and as it were attached to the 

 molecules of bodies, and, consequently, participates in the motions to 

 which the bodies may be subjected ; 2cl. The asther is free and inde- 

 pendent, and is not influenced by the motions of bodies ; 3d. Only a 

 portion of the aather is free, the other portion being attached to the 

 molecules of bodies, and participating in their motion. This latter 



