114 



Beccaria pub- 

 Jiihed a curious 

 work on the 

 refraction of 

 cryftals. 



Huygens mif . 

 taken in calling 

 the refraction 

 irregular j 

 as it uniformly 

 depends on the 

 internal ftruc- 

 ture of the 

 cryftal. 



Beccavia cut 

 rock cryftal in 

 different diiec- 

 tions i 



whence he ob- 

 tained certain 

 laws of their 

 refraction. 



MEMOIR OK ACHROMATIC CLASSES. 



It may not be improper here to give a brief abftract of a 

 very curious work, which Beccaria publifhed in Italian about 

 this time, as I learned from it the direction in which it was 

 neceflary to cut my prifms, to be certain of obtaining the ef- 

 fects I fought. Huygens, fays Beccaria, who treats very fully 

 on the double refraction of Iceland cryftal, adds, that rock 

 cryftal poflefles the fame property, but in a lefs fentible degree. 

 I faw that the refraction which Huygens calls irregular, bears 

 a conftant relation in the Iceland cryftal to its internal ftruc- 

 ture ; for the effect of this refraction is to divert the rays in the 

 direction of the faliant angles, which are equal in the whole 

 piece, and in each of its component parts. From infpecting 

 the figure of rock cryftal, I inferred its internal ftructure. In 

 fact, I coniidered it as fimilar to artificial cryftallizations. 

 formed by the aggretation of little laminae parallel to the faces 

 of the cryftals. Sufpecting Huygens of inaccuracy in avert- 

 ing, that prifms of rock cryftal had always a double refraction, 

 whatever their fections were, I cut a piece of rock cryftal, of 

 a very irregular figure, in the direction of its axis, and lb as 

 o divide two of its oppofite faces into two equal parts. I 

 cut another likewife in the direction of its axis, but lb that the 

 lection parted through the fummits of two oppofite angles. A 

 third afforded me triangular prifms, one of the faces of which 

 was one of the faces of the cryftal itfelf, while each of the other 

 two faces terminated on one fide at one of the contiguous an- 

 gles, and on the other at the axis of the cryftal. A fourth 

 was divided into equilateral prifms, two of the faces of which 

 were equally inclined to the axis, and the third was parallel 

 to a plane continued through the axis. From the obfervations 

 made with thefe prifms, I obtained a very ftmple law, of which 

 Newton fays nothing, and the contrary of which is advanced 

 by Huygens. 1. That the double refraction does not fubfi ft 

 in all the different prifms that may be cut from rock cryftal, 

 2. That the ray of light which traverfes rock cryftal in a place 

 perpendicular to the axis undergoes two refractions, is divide^ 

 into two, and gives two images, if not completely yet fenfibly 

 diftinct. 3. That this diftinctnefs of the two images dimi- 

 nifhes in proportion as the direction of the ray approaches to 

 that of the axis of the cryftal. 4. That the double refraction 

 and difperiion of the two images ceafe entirely, when the 

 direction of the ray is parallel to the axis, or nearly parallel to 



it. 



