PROCEEDINGS OF SOCIETIES. 107 



Acus are easily defined. The price of the condenser is eight 

 guineas, being only one guinea more than the old one of 100° 

 aperture. 



I have also to bring before the notice of the meeting, a 

 method which I have some time adopted for showing the 

 phenomena of the rings round the optic axes of crystals, 

 and which is chiefly my own invention. Mr. Woodward, the 

 author of a treatise on polarised light — Professor Potter, of 

 the London University — and Mr. Darkin, of Lambeth, have 

 all seen it ; and all agree that they have never seen the phe- 

 nomena so beautifully displayed. It consists — 



1. Of an eye-piece with lenses about the depth of those 

 usually put in the third eye-piece. There is no diaphragm 

 between the lenses, which are so adjusted that the field-lens 

 may be brought nearer to, or farther from, the eye-lens, as" 

 occasion may require, thus giving different powers and 

 different fields ; and when adjusted for the largest field it 

 will be full fifteen inches, and take in the widest separation 

 of the axes of the arragonite ; 



2. A crystal stage to receive the crystals, and of the usual 

 construction, into which is screwed a blue tourmaline ; 



3. A large Nichol's prism as a polariser ; 



4. A common single double convex three or four-inch 

 lens, set in the middle of a brass tube, long enough, when 

 screwed into the body of the microscope, to reach the pola- 

 riser, so that all extraneous light may be excluded. At the 

 bottom of this tube there is a blue glass, for the purpose of 

 giving white light when a lamp is used. The concave mirror 

 should be used with a bull's-eye condenser by lamplight ; 

 the condenser may be dispensed with by daylight. Messrs. 

 Powell and Leal and are well acquainted with the above 

 apparatus. 



The crystals best adapted to show the phenomena of rings 

 round the optic axes are — 



Quartz, a uni-axial crystal, one system of rings, no entire 

 cross of black, only the ends of it, the centre being coloured ; 

 and as the tourmaline is revolved, the colour gradually 

 changing into all the varied tints of the spectrum, one 

 colour only displayed at once in the centre ; 



Quartz, cut so as to show right-handed polarisation ; 



Quartz, cut so as to show left-handed polarisation — that 

 is, the one shows the same phenomena when the tourma- 

 line is turned to the right, as the other does when turned to 

 the left ; 



Quartz, cut so as to show straight lines ; 



Ca/c Spar, a uni-axial crystal, one system of rings, and a 



