618 



Transactions of the Society. 



end-faces are principal planes of section of the crystal, the optic axis 

 lying in them diagonally. The method of cutting the spar is a simple 

 modification of that adopted of late years hy Mr. Ahrens, which, while 

 gaining the maximum angular aperture, causes least waste of spar. 

 Fig. 73 shows another way of attaining the same end by use of a simple 

 divided rectangular parallelopiped of spar, the upper and lower faces 



Fia. 72. 



Fig. 73. 



of which are, as in the other form, principal planes of section. The 

 cone of useful rays is confined within the dotted lines. The analysers 

 can be used over either an A or B eye-piece. 



For showing the rings and brushes in crystal sections with an 

 ordinary Microscope, I have adopted the following arrangement, which 

 is quite as satisfactory as the much more expensive arrangements 

 which go by the name of Norremberg's apparatus for convergent light. . 

 The polarizer described above, together with the wide-angled Abbe- 

 Beck condenser, is fitted in its place in the substage. Ordinary 

 objectives are of little use for the purpose of showing the rings and 

 brushes. What is wanted is a very wide-angled objective of not very 

 high power. ^^ ot even the wide-angled apochromatic objectives now 

 in fashion are satisfactory. Instead of any of these, I employ a wide- 

 angled achromatic substage condenser (by Beck), which, being pro- 

 vided with the universal screw, can be removed from the adapter 

 which fits the substage and screwed into the lower end of the tube 

 instead of an objective. The eye-jiiece must be removed from the 

 draw-tube. To observe the rings and cross of a uniaxal crystal, or 

 the rings and brushes of a biaxal, the crystal slice is placed on the 

 stage, the substage condenser is screwed close up to it from below, 

 and the achromatic condenser in the tube is screwed down close upon 

 it from above. The observer's eye is placed over the open upper end 

 of the tube. Thus viewed, the rings and cross or rings and brushes 

 appear quite small in the contracted field of light. To magnify them, 

 the following very simple device is sufficient. A positive lens of any 

 focal length between IJ and 4 inches, according to the choice of the 

 observer, is placed about half-way down the tube. A spectacle glass 

 of 3-inch focus, snipped down with a pair of pliers and dropped into 

 the draw-tube so as to rest on the diaphragm, answers very well. 

 The arrangement shows the rings even of those biaxal crystals which 

 have very wide separation between the optic axes, such as gypsum 

 and topaz. 



