386 Proceedings of Philosophical Societies. [May, 



At the same meeting a paper by INIr. Hay was read, on certain 

 properties of tangents, of circles, and of trapeziums inscribed in 

 circles. 



On Thursday, the 21st April, a pper by Dr. Brewster, on the 

 optical properties of mother-of-pearl, was read. An outline of the 

 curious facts contained in this paper was given in the last number of 

 the Annals of Philosophy. 



When we look at the image of a candle reflected from the sur- 

 face of a piece of regular mother-of-pearl, ground but not polished, 

 we perceive at the distance of four or five degrees from the common 

 image a highly coloured image, the distance of which from the 

 common image increases with the angle of incidence. By polishing 

 the mother-of-pearl a new image exactly like the first, and obedient 

 to the same laws, is developed on the other side of the common 

 image. These optical properties may be communicated by pressure 

 to wax, cement, gum arabic, balsam of Tolu, realgar, tin foil, 

 the amalgam of bismuth, and even to lead. Hence it follows that 

 the optical properties of mother-of-pearl are owing to a certain 

 configuration of the surface, which cannot be removed by the finest 

 polishing. By examining the surface of mother-of-pearl by means 

 of microscopes, he found that it was composed of grooves similar 

 to the skin at the point of an infant's finger. These grooves are 

 very fine. The distance between them varies. Sometimes they 

 may be seen with the naked eye ; sometimes there are about 3000 

 in the inch. It is to this grooved structure that mother-of-pearl is 

 indebted for its optical properties. 



LINN^AN SOCIETY. 



On the 5th April there was read a tabular view of four classes of 

 animals, considered by Linnffius as constituting but one class, by 

 William Elford Leach, M. D, The principal object of this 

 paper is to call the attention of entomologists to examine into the 

 propriety of constituting a new class of animals to comprehend the 

 classes syngnatha and chilognatha of Fabrlcius, which Latreille 

 and Lamarck have arranged with the arachnides. 



On the 19th April a paper, also by Dr. Leach, was read, on the 

 distributicti of the Crustacea into orders, in which the entomocrasla 

 and myriapoda of Miiller are considered as forming sub-classes. 

 He proposes to divide the malacostraca into three orders, two of 

 which with pedunculated eyes are distinguished from each other by 

 the form and proportion of the tail : the third order, with sessile 

 eyes, he admits to be artificial. 



Some observations by the President on Brodel's remarks upon 

 the subject of Mr. Dickson's work on mosses, were also read. 



GEOLOGICAL SOCIETY. 



On the 21st January a letter from Mr. Grecnough was read, 

 relating the result of an examination made by himself, in company 

 with Mr. Irton and Mr, Buckiand, concerning the sand tubes that 



