108 PROCEEDINGS 01 SOCIETIES. 



black cross, which changes into a white cross, on revolving 

 the tourmaline, and the colours of the rings into their com- 

 plementary colours. 



Topaz, a hi- axial crystal, although it has two axes, only 

 exhibits one system of rings Avith one fringe, owing to the 

 wide separation of the axes ; the fringe and colours change 

 on revolving the tourmaline ; this is the case in all the 

 crystals ; 



Borax, a hi -axial crystal, the colours more intense than in 

 topaz, but the rings not so complete, only one set of rings 

 taken in, from the same cause as topaz ; 



Rochelle Salt, a bi-axial crystal, the colours more widely 

 spread, very beautiful, only one set of rings taken in; 



Carbonate of Lead, a bi-axial crystal, axes not much sepa- 

 rated, both systems of rings exhibited, far more widely spread 

 than those of nitre ; 



Nitre, a bi-axial crystal, axes very closely approximated, 

 colours intense and beautiful, the rings are also closely 

 united ; 



Arrar/onite, a bi-axial crystal, axes rather widely separated, 

 but both systems of rings exhibited, and decidedly the best 

 crystal for displaying the phenomena of bi-axial crystals. 



The field-lens of the eye-piece requires to be brought as 

 close as possible to the eye-lens, to see properly the pheno- 

 mena in quartz and arragonite; it must be placed at an inter- 

 mediate distance for viewing topaz, borax, Rochelle salt, and 

 carbonate of lead ; it must be drawn out to its fullest extent 

 to view nitre and calc spar. 



The powers of the miero-polariscope cannot be better 

 displayed than in the exhibition of the foregoing pheno- 

 mena ; there is nothing more beautiful, and few studies 

 more interesting and enlarging to the mind, than that of 

 light, whether common or polarised, which must be entered 

 upon, if the phenomena are to be understood. 



The crystal eye-piece, with an artificial tourmaline as an 

 analyser, will be found very useful for polariscope objects 

 generally. There is some spherical aberration, but the 

 largeness of the field far more than compensates for the same. 

 It docs best for those objects that require the two-inch 

 object-glass. 



1 have brought with me this evening two objects — a flea 

 and an insect named Clialii'er. They "ere mounted by one 

 of the members of this society, Mr. Farmer, of Hornsey ; and 

 ;ne remarkable for the clear manner in which the muscles 

 arc brought out, as also the Btriated fibre of the same ; and 

 which, at the instigation of our much-esteemed president. 



