264 M. G. Wertheim on the double Refraction 



All these substances are colourless, with the exception of the 

 heavy flints and some of the fluorides of lime, and among these 

 latter there are some possessing a tint so clear that the light, 

 after having traversed a considerable distance of them, might 

 still be regarded as sensibly white ; we have sought to neutralize 

 the tinting of the others by glasses coloured with the comple- 

 mentary tint. Another process consists in employing in the 

 first instance the substance itself as a coloured glass, and deter- 

 mining by the compression of a colourless glass the lengths of 

 the undulations of the light which it transmits. We shall see 

 immediately, that when once this length is known, it is easy to 

 calculate the weight which it would have been necessary to em- 

 ploy with white light. 



1 have called inactive alum an alum procured from a manufac- 

 tory at Muirancourt (Aisne), respecting the method of preparation 

 of which I have been unable to obtain any information ; very 

 limpid, not tarnishing in moist air, and preserving for years the 

 polish imparted to its surfaces, natural or artificial ; this aluin 

 possesses also the precious quality of being completely exempt 

 from laminar polarization, notwithstanding the notable quantity 

 of ammonia which enters into its composition. Even when a 

 piece of this alum is pressed to crushing, the fragments exert 

 no action on polarized light ; it also distinguishes itself from 

 ordinary alum by its conchoidal fracture. 



The numbers which refer to rock-salt do not merit the same 

 degree of confidence as the other numbers contained in the pre- 

 ceding table ; notwithstanding all the care which I have taken 

 to neutralize the pre-existent laminar polarization, and the per- 

 manent tinting which exhibits itself on the application of the 

 mechanical force itself, this cause of error, combined with the 

 imperfect transparency of the substance, has necessarily rendered 

 the determinations less sure and precise. 



I have cited in the foregoing table, among the crystals which 

 belong to the regular system, only the species and the individual 

 (if I may use the expression) which under the action of mecha- 

 nical forces behave as homogeneous or isotropic bodies*. 



Experimental Laws, 



1. The weight [necessary to be applied to produce a certain 

 double refraction) is independent of the height of the parallelopiped. 



This proposition, evident of itself, is confirmed by all the 

 experiments; to cite only one example, the value of P x is 



2 



the same for the two specimens of fluor-spar 21 (first experi- 



* See my notes on the artificial double refraction produced in crystals of 

 the regular system (Coiwp^es Henrftw, vol. xxxiii.p.57/,and vol.xxxv.p.276). 



