434 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The lens of longer focus is intended for use without the eye-piece, 

 to enable the observer to see the interference figures (generally only the 

 axial shadows being visible) by looking down the tube, when a 1/10 or 

 1/12 in. objective is used on very small crystals, in convergent light, 

 between crossed nicols. 



There is a weaker lens for the same purpose fitting into the top of 

 the tube, which can be used with a 1/4 or 1/6 in. objective by those who, 

 like myself, cannot see the figures without some aid. 



The eye-piece, when in use, turns with the polarizing apparatus. 

 It contains the usual cross wires, and has an adjustment to enable an 

 observer to focus the wires or the micrometer alluded to above. A 

 quarter-undulation plate of mica or a wedge of mica or quartz can be 

 pushed through the eye-piece at 45'^ to the direction of the cross wires. 



When the eye-piece is not in use its place can be taken by a fitting 

 which carries the analyser and the weakest lens alluded to above. The 

 condenser of the instrument consists of a lens screwed upon the top of 

 the polarizer, which slides up and down. The lens is suitable for all 

 objectives up to the 1/2 in. For higher powers and interference figures 

 a small hemispherical lens is fitted into the stnge and can be pushed into 

 the axis of the instrument when required. The upper surface grazes the 

 lower side of the glass slip carrying the object. The focusing is done 

 by raising or lowering the polarizer carrying the aforesaid lens. This 

 arrangement is found to work admirably. 



The rotation of the eye-piece and polarizing apparatus is measured 

 on a circle graduated to degrees, but by using a pocket-lens a good 

 reading to half a degree can be obtained, and a fair reading to a quarter 

 of a degree, nearer than which extinctions or angles cannot be measured, 

 even under the most favourable circumstances. 



When the indicator is at zero on the graduated circle the cross 

 wires are upright and horizontal as the observer looks into the instru- 

 ment. If the polarizer is in its catch any suitable crystal with straight 

 extinction will be at the maximum darkness when parallel to either 

 cross wire. If Klein's plate be now pushed into the tube of the Micro- 

 scope, and the analyser turned in its fitting till the crystal and the field 

 are of one uniform warm blue tint, it will be found that the nicols are 

 accurately parallel. The nicols can then be turned parallel to one 

 another by the toothed wheels. This is almost the only use I have found 

 for the Klein's plate. I wished to put it aside altogether from a student's 

 instrument, but Mr. Swift informed me that a Microscope, to be used even 

 occasionally for petrological investigations, cannot be sold without such 

 a fitting, buyers requiring it though they do not appear to make any use 

 of it. It must be regarded as part of the little lumber which it seems 

 this instrument must possess. 



If the mineral with straight extinction is not lying parallel to either 

 cross wire, it will be found that when the wheels are turned the crystal 

 will be extinguished when either of the wires becomes parallel with it. 

 If the mineral has an oblique extinction a reading of the circle must be 

 made when one or other wire is parallel to one of the edges or lines of 

 the crystal, and another reading after continuing the rotation till the 

 maximum extinction is attained. The rotation is then continued through 

 45°, and a mica- or quartz-wedge pushed through the slot in the eye- 

 piece to ascertain the direction of the major or minor axis of elasticity 

 and its inclination to the edge or line if desired. 



