126 ANNUAL OF SCIENTIFIC- DISCOVERY. 



made previously with air in motion, and I have demonstrated that the 

 motion of the air does not produce any sensible displacement in the 

 motion of the fringes. In the circumstances in which that experiment 

 was made, and with a velocity of 25 metres a second, which was that 

 of the motion of the air, it is found that according to the hypothesis 

 by which the aether is considered to be carried along with the bodies, 

 the double displacement ought to be 0.82. According to the hypothe- 

 sis of Fresnal, the same displacement ought to be only 0.000465, that 

 is to say, entirely imperceptible. Thus the apparent immobility of the 

 fringe in the experiment made with air in motion is completely in 

 accordance with the theory of Fresnel. It was after having demon- 

 strated this negative fact, and while seeking for an explanation 

 by the different hypotheses relating to the tether in such a way as to 

 satisfy at the same time the phenomena of aberration and the experi- 

 ment of M. Arago, that it appeared to me to be necessary to admit 

 with Fresnel that the motion of a body occasions an alteration in the 

 velocity of light, and that this alteration of velocity is greater or less 

 for different mediums, according to the energy with which those 

 mediums refract light, so that it is considerable in bodies which are 

 strongly refractive, and very feeble in those which refract but little, 

 as the air. It follows from this, that if the fringes are not dis- 

 placed when light traverses ah* in motion, there should, on the con- 

 trary, be a sensible displacement if made with water, the index of 

 refraction of which is very much greater than that of air. The suc- 

 cess of the experiment seems to me to render the adoption of Fresnel's 

 hypothesis necessary, or at least the law which he found for the 

 expression of the alteration of the velocity of light by the effect of mo- 

 tion of a body ; for, although that law being found true may be a very 

 strong proof in favor of the hypothesis of Avhich it is only a conse- 

 quence, perhaps the conception of Fresnel may appear so extraordi- 

 nary, and in some respects so difficult to admit, that other proofs and a 

 profound examination on the part of geometricians will still be neces- 

 sary before adopting it as an expression of the real facts of the case. 

 Comptes Rendus, Sept. 1851. 



HELIOCHROMY, OR DAGUERREOTYPES IN COLORS. 



FROM the time of the discovery of the Daguerreotype, unceasing ef- 

 forts have been made by scientific men to produce photographic pic- 

 tures in which the various natural colors of objects might be faithfully 

 represented. M. Becquerel, in experiments made in 1849-50,* suc- 

 ceeded so far, as to be able to transfer to a metallic plate the colors 

 of the prismatic spectrum in great brilliancy. These colors, however, 

 could not be rendered permanent, and soon disappeared. During the 

 past y^ear, however, the problem has been solved by M. Niepce, a dis- 

 tinguished French photographer, and nephew of the celebrated Niepce, 

 who, in connection with Daguerrc, discovered the Daguerreotype. The 

 process by which this has been effected, has received the name of Helio- 

 chromy, or sun-painting, and although still imperfect, is of the highest 



T i J I__1_J_M^^^^ 



*Sce Annual of Scientific Discovery, for 1851, p. 150. 



