226 Proceedings of Philosophical Societies. [Maecm 



account of these researches of Biot in their order, beginning 

 with the first memoir, which was read to the institute on the 

 1st of June, t8l2 ; and every page of which was marked by M, 

 Pelambre, perpetual secretary. 



M, Biot, io the first place, contrived an apparatus with which 

 all the phenomena may be exactly observedj and the angles 

 measured. He makes the white light from the clouds fall 

 upon a well-polished black glass, under, such an angle that it 

 is completely polarized by reflection. He makes the reflected 

 ray pass through the tube of the telescope of a repeating circle,, 

 deprived of its glasses. The edge of the circle is placed verti- 

 cally, and parallel (o the plane of reflection. The upper extre- 

 mity of this tube is enveloped in a circular drum, which turns 

 stiffly round it. Its circumference is divided into ] 6 parts, each 

 of which, of course, corresponds to an angle of 22° 30\ At 

 the two opposite extremities of the same diameter are two 

 branches of copper parallel to the axis of the tube, between 

 which there is a circular plate of copper, which turns freely 

 round an axis perpendicular to the two branches. This plate 

 itself carries a ring, which turns freely round its surface, and 

 round the axis of the tube. All these different motions rnay be 

 regulated and stopped by screws. The crystalline plate, on which 

 the observation is to be made, is placed upon this ring. It is 

 evident, 1. That by turning the ring round its centre, upon the 

 plate which carries it, the axis of the crystalline plate may be 

 directed so as to form any angle whatever with the plane of 

 polarization. 2. That by turning the plate of copper which 

 supports the ring the crystalline plate may be inclined at pleasure, 

 with respect to the polarized ray. 3. That by turning the drum 

 round the tube which it envelopes, the plane of incidence of the 

 ray upon the crystalline plate may be placed at all possible azi- 

 muths round the plane of polarization. The crystalline plate 

 may then be presented to the ray in all possible positions. The 

 angles which determine these positions are measured by the 

 divisions on the apparatus. The incidence of the ray upon the 

 surface of the plate is likewise measured by bringing the plane 

 of incidence into the plane of the edge of the circle, and raising 

 the tube till the surface of the plane becomes exactly horizontal^ 

 which is ascertained by a very sensible level. 



The first experiments of M. Biot were made with plates of 

 sulphate of lime. The easy divisibility of this substance, and 

 the possibility of reducing it into thin, equal plates, with surfaces 

 perfectly parallel, and finely polished, makes it exceedingly proper 

 for exact observations. M. Biot began by endeavouring to dis- 

 cover the direction of the axis of double refraction. The pri- 

 mitive form of this substance assigned by Haiiy is a rectangular 

 four-sided prism, whose bases situated in the plane of the plates 

 ?\re parallelograms with angles of 113^ 7' ^.S'' and 66'^ 52' 



