844 



M. 6. Wertheim on the double Refraction 



side of each of the parallelopipeds. Put in vibration by means 

 of a bow, the two ends being free, each plate gave its fundamental 

 tone and some of its harmonics. Experiments have been recently 

 published which tend to cast some doubt on the correctness 

 of this method, and on the certitude of the results derived 

 from it. But by means of experiments on strips of copper of 

 various lengths and thicknesses, which I have caused to vibrate 

 transversely, after having previously determined the elasticity by 

 means of direct elongation, I have assured myself anew of its 

 accuracy ; only it is necessary to take care not to fasten one of 

 the ends of a short plate when transversal vibrations are required : 

 the fastening of the end causes, as is known, errors so consider- 

 able as to alter completely the results of the experiments. 



The values of E which have been obtained by this method, 

 have been afterwards divided by the coefficient of correction! '05*, 

 in order to find those>which would have been found by the way 

 of elongation. 



I have caused a plate to be cut for each of the parallelopipeds ; 

 I have hence three plates of rock-salt from different sources, 

 several of fluor-spar, &c. For plates of the same kind, the co- 

 efficients of elasticity differ so little among themselves, that I 

 have only found it necessai*y to inscribe the means in the follow- 

 ing table, reserving to myself to publish the numbers themselves 

 in a special memoir on the elasticity of crystals. 



* Ann. de Chim. et de Phys. 3 sir. vol. xxxi. p. 39. 



t The density 2*329 of the plate glass employed by M. Dutirou, differs 

 too much from that of my glass to permit of my assuming the index found 

 by this physicist. 



X Having been able to procure only a single cube of the glass, I have 



