242 M. G. Wertheim on the double Refraction 



The apparatus (Plate II. figs. 1 and 2) which I have used to 

 produce the mechanical eflfect is composed of two pieces for com- 

 pression A, and A^, of one piece for traction B, and two electro- 

 magnetic spirals, which form part of the apparatus of M. Ruhm- 

 korff, and which carry, one the polarizing Nichol C, and the 

 other the doubly refracting prism D. All these pieces are fixed 

 upon a support of oak, furnished with notches so as to give pass- 

 age to the hooks destined to bear the weights, and with grooves 

 q in which slide the prolongations of the cores of the spirals. 



It is easy to undeffetand the dispositions of the presses which 

 we shall name weight presses, fig. A; each of them is com- 

 posed of a vertical stirrup a, fastened down to the bench, and of 

 the two horizontal plates b, parallel to each other and to the 

 horizontal portion of the stirrup. This latter is pierced with 

 two apertures, in which slide with gentle friction two cylindrical 

 stems c, which connect the two moveable plates, the upper one 

 of which is placed on the piece p submitted to compression, 

 while the lower one is furnished with a ring d for the application 

 of the charge. The whole press is in bronze, 2 centimetres thick 

 and 3 centimetres wide, and hence sufficiently solid to prevent 

 any of its parts from suffering a sensible flexure under the action 

 of the charge, which never exceeded 600 kilogrammes. The 

 parallelopiped ^ is placed on the horizontal portion of the stir- 

 rup, so that its centre is in the prolongation of the axes of the 

 prisms; the two horizontal sui'faces of this piece, those that 

 receive the pressure, are furnished each with a plate of vulcanized 

 india-rubber, and several doubles of thin pasteboard. In this 

 manner the pressure distributes itself in a uniform manner over 

 the whole parallelopiped, since the upper plate cannot deviate 

 from the horizontal line; and if, notwithstanding these pre- 

 cautions, some defect of parallelism should exhibit itself, it can 

 always be remedied by means of the screws r. 



One of these weight presses Aj is destined for the employment 

 of heavy charges, which are placed in the -chest E furnished with 

 a support at its base ; it is only necessary to lower this support 

 to cause the entire charge to be carried by the bar e, which is 

 suspended from the ring d. 



The other press A^, the use of which we shall explain imme- 

 diately, is destined to receive comparatively light charges only ; 

 it therefore carries only a square bar/, furnished with cross-bars 

 at its two extremities, which reach over the bench, and receive 

 each one-half of the charge. 



The apparatus for extension, fig. B, is composed of a support 

 of cast iron g, larger and more elevated than the stirrups of the 

 presses, and of the two truncated bronze cones, the superior one 

 of which carries a strong vice i, while the inferior one k is fur- 



