12G 



PROCEEDINGS OF THE SOCIETY. 



MEETING 



Held on the 18th of December, 1907, at 20 Hanover Square, W. 

 Mr. Conrad Beck, Vice-President, in the Chair. 



The Minutes of the Meeting of the 20th of November, 1907, were 

 read and confirmed. 



The List of Donations to the Society since the last Meeting, exclu- 

 sive of exchanges and reprints, was read, and the thanks of the Society 

 were voted to the donors. 



From 

 Bernard Rawitz, Lehrbuch der Mikroskopischen Technik.\ rp 7 p , r , 



(8vo, Leipzig, 1907) f ** ^ u0l/isner - 



Eug. Warming, Dansk Plantevsekst. 2 Klitterne, F<prste\ m, A .-, 



Halvbind. (8vo, Copenhagen, 1907) J ine manor - 



^-in. Objective by Andrew Ross, date about 1842 . . . . Mr. J. E. Ingpen. 



Mr. Eustace Large described a number of slides of natural twin- 

 crystals of selenite exhibited under Microscopes in the room. The way 

 in which the specimens had been prepared and the effects produced by 

 the varying angles at which the twin-plane cut the cleavage-plane were 

 further illustrated by diagrams and models. Some large reflecting 

 polariscopes with horizontal stages were also exhibited, and a description 

 of these was appended to the paper. 



Mr. Large said : " I propose to assume a general knowledge of the 

 action of polarised light, and will only briefly refer to two points, viz. : 

 1. That the thickness of a plate of selenite, or mica, determines the 

 particular wave-length that, by interference, will be cancelled ; leaving 

 the residue of wave-lengths, of the particular light we may be working 

 by to combine and form the actual colour seen, which colour will be com- 

 plementary to that cancelled ; and (2) That if an even plate be cut in 

 half, and one half placed over the other parallel, the colour will be that 

 of a plate double the thickness, but if one be crossed at right angles on 

 the other the action in the one will exactly counteract that in the other, 

 and darkness will be restored, that is, assuming the nicols to be crossed, 

 giving a dark field. Consequently, if two pieces of unequal thickness 

 be crossed the colour will be that of a plate equal to their difference in 

 thickness. Selenite is the natural crystallised form of gypsum, and con- 

 sists of the metal calcium + sulphuric acid + 2 equivalents of water. If 

 1 part of the water be driven off by heat plaster of Paris remains. If 

 both, the plaster produced will not " set." The crystals occur naturally 

 of all sizes. They do not appear to have been produced artificially of 



