848 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



scope with the alterations necessary to fit it for lithological examina- 

 tions. The coarse adjustment is effected by the usual rack-and-pinion, 

 and the fine by a micrometer-screw with a divided milled head, repre- 

 senting thousandths of an inch, for the approximate measurement of 

 sections, &c. The stage is divided on the edge to degrees, and has a 

 vernier reading to 10', adapting it for use as a goniometer or for 

 stauroscopic measurements, &c. It rotates concentrically with the 

 optic axis, and to compensate for any slight variation in centering 

 there is a centering nose-piece. Immediately above the latter is a 

 Klein's quartz plate fixed on an arm by means of which it can be 



Fig. 153. 



Fig. 154. 



/ 



turned up out of the field with great facility as 

 shown in Fig. 153. One of the three milled heads 

 at the end of the body-tube effects this movement. 

 The other two milled heads are for centering 

 the objective, and their action is shown in the same 

 figure. 



The analyzer rotates freely over the eye-piece 

 (Fig. 153), and has an index which by a divided 

 plate on the draw-tube allows it to be read 

 in any position and recorded. Between the analyzer and the eye- 

 piece is placed a plate of calc-spar cut at right angles to the optic 

 axis for stauroscopic measurements. The eye-piece has cross cob- 

 webs. The rotating polarizer also has a divided circle to register 

 its position, and it is fitted into a swinging arc which is in contact 

 with the bottom of the stage, thus excluding any false light. By 

 means of a hinge it can be instantly turned away from the stage (in 

 the way shown in Fig. 154) when ordinary illumination is required. 

 A condenser of large aperture, fitting into the tube of the polarizer 

 above the prism, is intended for the examination of the interference 

 brushes and rings in crystals with convergent light. This also is 

 easily removed when not required. When the instrument is used 

 for this purpose a lens is screwed into the lower end of the draw- 

 tube. 



The instrument, though specially constructed for the study of 

 rock sections, can be used for any other work. It is only necessary 

 to remove the analyzer and polarizer. The fitting on the swinging 

 arm which carries the polarizer will take any other substage apparatus 

 such as parabola, achromatic condenser, &c. 



