286 SENARMONT ON THE OPTICAL CHARACTERS OF 



that of maximum and sometimes that of minimum elasticity, or 

 in which the directions of the ordinary and extraordinary rays 

 are mutually inverted. 



But as the transition from one optical character to the oppo- 

 site appears to be gradual and continuous, it must be possible 

 to obtain mixtures in which the double refraction would be de- 

 stroyed, or rather which would behave for one extremity of the 

 spectrum, like hyposulphate of lead, by refracting the ordinary 

 ray less than the extraordinary ray, while they would act for the 

 other extremity of the spectrum like hyposulphate of strontia, 

 refracting the ordinary ray more than the extraordinary ray, 

 and which, lastly, would not present any double refraction for 

 the intermediate part of the spectrum, the ordinary and extra- 

 ordinary rays remaining united. 



I have, in fact, after several attempts, obtained crystals which 

 present under Amici's polarizing microscope a violet field with 

 a black cross. The rings are so much expanded by the almost 

 total obliteration of the birefractive power, that the microscope 

 includes only the second obscure ring. But as the field of the 

 first bright ring presents a uniform violet colour, instead of a 

 white of the first order, even when white light is employed, it 

 follows that all the elements of this light except the violet must 

 be extinguished in the analyser. They have therefore retained 

 their original polarization, and consequently have not experienced 

 double refraction. 



Other crystals presented, instead of the violet, a blue colour ; 

 as the proportion of strontia and lead salts varied, another part 

 of the spectrum escaped double refraction. 



It would be interesting to ascertain whether, in this trans- 

 formation of optical characters, the dispersion of the colours does 

 not suffer in the ordinary and extraordinary rays some inversion 

 of direction or magnitude ; but as this inquiry would necessitate 

 the use of large and pure crystals, I am compelled to postpone 

 it until a later period. 



Crystals of Potash and Ammonia Seignette Salt, 



Sulphate and chromate of potash combine together, forming 

 crystals which are very well developed, although too small to 

 admit of their optical characters being readily studied. This 

 may however be easily done with crystals consisting of ammo- 



