183 



Monday, 21st March 1853. 



lliGHT Rev. Bishop TERROT, Vice-President, in the Chair. 



The following Communication was read : — 



On Circular Crystals. By Sir David Brewster K.H.,D.C.L., 

 F.R.S., V.P.R.S.E, Associate of the Institute of France. 



The author, after mentioning Mr Fox Talbot's observation, in 

 1836, of circular crystals from a solution of borax in phosphoric acid, 

 stated, that about twenty years before Mr Talbot's paper was pub- 

 lished, he had obtained circular crystals from oil of mace, and from 

 a mixture of that oil with tallow or rosin. These circular crystals 

 are groups of radiating prisms, in optical contact, so as to appear like 

 individual crystals. Viewed by polarized light, they exhibit four 

 luminous sectors, separated by a rectangular black cross, which 

 often has its arms so divergent, as to form four dark sectors. The 

 arms of the cross are parallel and perpendicular to the plane of pri- 

 mitive polarization. When a bright disc of ordinary light was looked 

 at through these circular spots, there was seen a halo, or two halos, 

 produced by the crystals of the oil of mace. In the case of two halos, 

 polarized light shewed two sets of four luminous sectors, as far apart 

 as the halos. The halos were, in fact, double, being the two images 

 produced by the double refraction of the elementary crystals. In 

 pursuing the inquiry, the author found that the phenomena were caused 

 by circular crystals or groups, varying from invisibility to the 2 o o^h 

 or 3^0*^ ^^ ^^ i^^^ ^" diameter, and when of this size, exhibiting 

 beautiful luminous sectors in polarized light. Circular crystals are 

 easily distinguished from those which exhibit true quaquaversus polari- 

 zation, by using a plate of selenite, which, with the circular crystals, 

 produces spots or sectors of two different colours, one a little lower, 

 the other a little higher than the tint of the selenite. 



He next examined a number of substances which yield circular 

 crystals, particularly the lithoxanthate of ammonia (a salt formed by 

 the action of ammonia on xanthic oxide), which yields them with more 

 facility and certainty than borax. Out of more than 300 substances, 

 he found upwards of 70 which yielded circular crystals, about thirty 



