temporarily produced in Isotropic Bodies. 249 



This table has served for all the experiments with white light ; 

 the most sensible tints have been preferred, those which are 

 found on the limits of the rings. 



But it is not always possible to push the experiments to so 

 great a difference of path, partly because of the two great di- 

 mensions of the piece, and partly because the substance will 

 not support a notable compression without being bruised, cloven, 

 or at least without being optically changed. This last alteration 

 is observed in several crystallized bodies, and among others in 

 rock-salt. It consists in a separation of the laminse of the cry- 

 stal, which, though invisible to the eye, manifests itself in polar- 

 ized light by permanent chromatic phsenomena, of such an inten- 

 sity that they often predominate over the temporary effect of 

 the charge*. 



In the experiments by means of traction, we are equally 

 obliged to stop at small charges on account of the fragility of 

 the mastic ; it was therefore indispensable to be able to measure 

 small differences of path with a precision greater than that which 

 appertains to the determinations made by means of the pale and 

 not very sensible tints which are found at the commencement 

 of the first ring. 



This object has been attained by the simultaneous employ- 

 ment of two pressures, or of one pressure and one traction. We 

 place under the press Aj, either the piece of crown glass which 

 we have just employed, or another piece for which the charges 

 producing the colours inscribed in our table have been pre- 

 viously determined, and compress it until the teinte sensible 

 appears. We then place the piece to be examined either in the 

 press A^, or in the apparatus of extension B, and apply to it the 

 feeble charge which it can carry without inconvenience. Look- 

 ing now across the two pieces, we see that the teinte sensible has 

 disappeared, and that it is replaced by a colour, more elevated, if 

 the second piece be compressed, and by a colour inferior in the 

 order of tints if the piece be elongated. In order to produce the 

 teinte sensible, it is necessary, in the first case, to remove from the 

 press A J a charge equivalent to that which has been applied to 

 A^, and, in the second case, to add a charge equivalent to that 

 which acts on B. We see that the press Aj then acts as a veri- 

 table compensator, the sensibility of which may be regulated at 

 pleasure by placing in it parallelopipeds which require charges 

 more or less considerable. 



We may operate in a different manner by comparing the 



traction with the pressure; first, we apply the traction, and 



afterwards seek the charge which it is necessary to apply to Aj 



in order to reduce the difference of the paths to zero. This 



* Comptes Rendus, vol, xxxiii. p. 577. 



