382 Cambridge Philosophical Society. 



Specimens of well-manufactured New Zealand flax were in the 

 Library, with various chemical apparatus, &c. 



The meeting was then adjourned over two Fridays, to the 15th 

 of April. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



'A meeting of this Society was held on Monday evening, Fe- 

 bruary 21, Dr. F. Thackeray, the Treasurer, in the chair. Various 

 books were presented to the Society, among which were three 

 volumes of the Correspondance Mathematique et Physique, pub- 

 lished by M. Quetelet, of Brussels, and presented by him ; Dr. 

 Morton's Travels in Russia, from the author, and a Russian Dic- 

 tionary presented by the same gentleman ; Mr. Jones's new work On 

 the distribution of Wealth, from the author ; The second edition 

 of the first volume of the Translation of Niebuhr, from the transla- 

 tors. The following presents to the museum were also announced : 

 several skins of birds and a collection of insects from China, 

 presented by the Rev. G. Vachell ; a collection of foreign insects, 

 by J. G. Children, Esq. ; and two specimens of Charr from Wales, 

 by W. Yarrell, Esq. A Daniell's hygrometer was presented by 

 R. W. Rothman, Esq. Fellow of Trinity College. W. Swainson, 

 Esq. well known as an ornithologist, was elected an honorary mem- 

 ber. A paper was read by Professor Airy, " On the nature of the 

 rays formed by the double refraction of quartz;" of which the fol- 

 lowing is an abstract : 



It is well known to those who have followed the recent discoveries 

 respecting the properties of light, that the phaenomena exhibited 

 by quartz are very different from those of any other substance of 

 similar crystalline character as for instance, calc spar. Thus, when 

 exposed to plane-polarized light, a plate of calc spar exhibits a 

 series of rings, of which the colours commence from Newton's black 

 at the centre ; and these rings are intersected by a black cross : 

 quartz, on the other hand, displays a series of rings, the central 

 point of which exhibits a colour different according to the thickness 

 of the plate : there is no cross, but at a distance from the centre, 

 rudiments of black brushes begin to appear. Again, in the case of 

 calc spar, on turning the analysing plate, the rings change in colour, 

 but are always circular, and of unchanged dimensions. On turning 

 the analysing plate in the experiment with quartz, the rings become 

 square figures, with a curious defect of symmetry, and dilate or 

 contract continually. If we put together a plate of right-handed 

 and a plate of left-handed quartz in the same apparatus, we obtain 

 a most singular and beautiful appearance, consisting of four co- 

 loured spirals cutting a number of concentric circles. 



On exposing these substances respectively to light circularly- 

 polarized, the appearances are still more remarkable : calc spar ex- 

 hibits rings dislocated at each quadrant, with a gray cross ; while 

 the colours in quartz are seen in the form of two spirals inwrapping 

 each other, with no black or gray cross. 



Professor Airy, after describing these phaenomena, the most 

 striking of which are new, proceeds to state and develop the hy- 

 pothesis 



