temporarily produced in Isotropic Bodies, 243 



nished with a ring /, destined to carry the hook m and the 

 weight-chest E. The piece/?, which is submitted to extension, 

 is fixed with red mastic to the bases of these two cones, which 

 has the double inconvenience of rendering the experiment deli- 

 cate, since the mastic is detached by the least transverse shook, 

 and of limiting its extent. This mastic scarcely supports, for the 

 square centimetre, a traction of 50 kilogrammes. However, this 

 mode of fixation is the only one admissible ; for the vice, or any 

 other mechanical appliance, necessarily exerts unequal pressures 

 in different directions, which give rise to the exhibition of colours 

 of the same nature as those we would observe, but so irregularly 

 distributed as completely to conceal the principal phsenoiaaenon. 

 The vice i of the superior cone is maintained vertical by means 

 of the traverse n, and its screw o serves to place the centre of 

 the piece p in the axis of the apparatus. 



These three supports are usually fixed on the bench ; how- 

 ever, when it is desired to bring into simultaneous action both 

 mechanical and magnetical forces, two of the supports are raised 

 so as to leave only the piece A or the piece B, according as we 

 operate by pressure or by traction. 



In causing the spirals to glide in their grooves q, they are 

 made to approach as nearly as possible to the piece to be sub- 

 mitted to their action ; they are fixed in this position by means 

 of the screws F, and against their bases the two pieces of soft 

 iron s are applied so that their cores constitute a single electro- 

 magnet. It is doubtless unnecessary to remark that an interval 

 ought always to exist between the piece jo and the poles, suffi- 

 ciently large to prevent the poles from moving towards each 

 other when the current is excited, and clasping the piece between 

 them, which would introduce a mechanical compression in the 

 direction of the axis of the piece. The same motive has caused me 

 to reject the iron and steel presses which I had first employed. 



In the prolongation of the optic axis of the apparatus is placed 

 a plate of white Sevres porcelain Gr, strongly illuminated by 

 white light, by the light of the salt-lamp, or else by a Carcel 

 lamp, the rays of which, before arriving at the Nichol, have tra- 

 versed glasses or liquids sensibly monochromatic. It would 

 doubtless be better to operate with direct white light, and with 

 coloured rays well defined by their vicinity to a line of the spec- 

 trum ; but this has not been possible, owing to the situation of 

 my laboratory and the long duration of each experiment. 



Supposing now, to fix the ideas, that a cube of a transparent 

 substance, colourless and isotropic, is placed in the press, that 

 the principal sections of the Nichol and of the doubly refracting 

 prism are parallel to each other, and enclose an angle of 45° with 

 the vertical, and that white light is made use of. As long as 



R2 



