F. F. Wright — Measurement of Extinction Angles. 373 



Fig. 10. 



Fig. 10. — Mikroskop-polymeter of Yoigt and Hochgesang carrying wedge 

 bolder H and also device described below for bolding small crystals. The 

 wedge bolder in turn carries a cap nicol, K, and a Ramsden ocular in tbe focal 

 plane of wbicb tbe wedge, W, is introduced. Tbe crystal-bolding device con- 

 sists of the following parts : S. tbe stand ; C, graduated vertical circle with 

 1° divisions: A, centering plate; B, ball and socket adjusting device; D, 

 crystal holder ; L, plano-concave lens for holding drop of liquid of same 

 refractive index as crystal and held by stand, E. 



The mikroskop-polymeter is a useful instrument in many ways but for 

 accurate work tbe mechanical workmanship on the same leaves much to be 

 desired. The fine adjustment screw is practically worthless for even approxi- 

 mate readings, the accurate adjustment of tbe different circles to the common 

 optic axis is not possible, and even if once accomplished does not remain in 

 adjustment : the divisions on the different verniers are inaccurate ; many 

 other defects have been felt by tbe writer and several of them remedied in 

 the workshop of the Geophysical Laboratory. — A stop diaphragm M has been 

 introduced into tbe upper tube, also an optic a«ial angle reflector. R (compare 

 F. E. Wright, this Jour., x:cii, 19-20, 1906); a holder for the combination 

 wedge is shown at T. which fits on tbe objective supporting arm and is 

 revolvable about the axis, an arrangement which has proved advantageous 

 in place of revolving tbe microscope stage when the interference plates are 

 inserted.— The fine adjustment screw P lias been lengthened so that it can 

 be reached from either side of the microscope. Tbe base of the stand at N 

 has been milled out to allow space for tbe reflector when the condenser lens 

 is lowered. 



