960 SUMMARY OF CURRENT RESEARCHES RELATING TO 



claimed to be " a valuable addition to a lens, while not affecting the 

 corrections or interfering with the performance of the objective in 

 any way. The plan will especially commend itself to all workers 

 who leave their objectives attached to the stands, as dust is sure to 

 find its way to them, even under glass shades." 



This device was adopted by the late F. A. Nobert many years 

 since. 



Swift and Son's G-oniometer Stage.— This instrument (fig. 156) 

 has been constructed by Messrs. Swift and Son for use with their 



petrological Microscope. 



YiQ, 156. It consists essentially of a 



^^^^^^^ pair of forceps attached to 



^^^^^^^^^^^. ^ pointer moving round a 



/^^^^ il^^^^\. graduated semicircle. It is 



^m ^■:-<JMC^^^^kr- - fj^^ used as follows to deter- 



£y -.^^Wl^^^^^^^^^^i»^Mr mine the separation of the 



j^ ^^^^^ ^ i I^^^^J^^fe ^^^y ^ optic axes in biaxial crys- 



f oswFTi son 7^ ^ P<D LONDON Q'^^ T A soction of the mineral 



~^ cleaved or cut perpendicu- 



^jl \^ ~" line ig placed in the forceps 



and the apparatus adjusted 

 on the stage of the Microscope, so that the line joining the optic axes 

 is inclined 45 per cent, to the crossed Nicols which are set parallel to 

 the cross wires of the eye-piece, whilst the same line is at right 

 angles to the direction in which the forceps point. The pointer is 

 then turned till the darkest part of one of the " brushes " covers the 

 intersection of the cross wires, when a reading of the scale is made. 

 The pointer is afterwards turned in a contrary direction till the 

 darkest part of the other brush covers the intersection of the cross 

 wires, when a second reading of the scale is made. The difference 

 between the two readings gives the apparent angle in air. The angle 

 in oil or other liquid can be determined in the same way by setting 

 the Microscope horizontally and adapting a small glass cell filled 

 with oil or other liquid, but in this case it is requisite to use an eye- 

 piece provided with a Nicol which rotates so as to allow of the 

 polarizer and analyser being set at 45° to the vertical direction of the 

 forceps. 



Very small sections of minerals may be attached by wax to the 

 point of the forceps or to a needle fixed in their place. 



Hartnack's Goniometer-stage. — The stage shown in fig. 157 has 

 priority of date (by several years) over the preceding. 



The base-plate lies on the stage of the Microscope, to which it is 

 clamped by the small screw in the angle-piece below the semicircle, 

 two other similar angle-pieces, but without screws, being fixed to the 

 opposite and one of the remaining sides. On the base-plate are two 

 glides moved by the milled heads at the back and right-hand side of 

 the fig., giving lateral motions in two rectangular directions. The 



