370 SUMMARY OF CURRENT RESEARCHES RELATING TO 



by a lever C H and bridge C B, and that it may be thrown in and out of 

 action at any elevation of the illuminating apparatus. This hinged 

 condenser may be supplied in two forms, one of the customary aperture 

 of 120°, the other having an aperture corresponding to that of a wide- 

 angle lens of N.A. 1*48. As the apertures of the objectives advance it 

 will be found necessary to centre the hinged condenser after the inter- 

 change, if the available polarised field is to be fully used. This 

 adjustment is effected by a horizontal ring recessed into the disk g and 

 carrying the hinged condenser, whilst two screws, C C (figs. 87 and 88), 

 and a copper spring serve to centre it accurately with respect to the axis 

 of the Microscope. The movement of the hinged condenser is limited 

 by a screw. In all, there are six methods of illumination attainable. 

 1. After removal of the entire illuminating apparatus the object may 

 be illuminated either direct or with the aid of the mirror, according to 

 the inclination of the body. 2. Illumination maybe produced by means 

 of the iris diaphragm and the rack-and-pinion only. 3. Illumination 

 with the iris diaphragm, the lower condenser and the rack-and-pinion 

 motion. 4. Illumination, after folding back the upper condenser, removing 

 the slider S and substituting for the polariser a wide-angle Abbe con- 

 denser. In this case the rack motion serves for focusing this condenser, 

 the angle of the illuminating pencil being adjustable either by a 

 vertical movement of the condenser or by means of a wheel diaphragm, 

 which may be attached to the lower end of the condenser mount. The 

 wheel diaphragm forms part of a simple apparatus provided for the 

 production of oblique illumination, such as is employed in determining 

 refraction by Schroeder v. d. Kolk's method of envelopment. 5. Illu- 

 mination by parallel polarised light in conjunction with the entire 

 illuminating apparatus, excepting the hinged condenser ; and (6) finally, 

 illumination by convergent polarised light with the assistance of the 

 hinged condenser. 



The observation tube consisting of : the objective, objective clutch, 

 compensation slit, objective centring device, and sliding analyser are 

 placed at the lower end of the tube ; whereas the Bertrand lens and the 

 eye-piece are contained within the draw-tube. Full particulars of all 

 these parts are given by the author. 



To render the Microscope available as a focimeter, a vertical scale 

 divided into \ mm. is attached to the left of the tube, so as to slide 

 along a vernier on the intermediate fitting B g (fig. 88), above the milled- 

 head of the coarse-adjustment, which renders it possible to read to 

 -^ mm. 



The author adds and explains sectional drawings illustrating (1) the 

 ray-path with parallel light in an ordinary Leitz Microscope ; (2) the 

 ray-path within the petrological Microscope of convergent polarised 

 light. 



Fig. 92 shows the revolving slide-diaphragm, which is affixed to the 

 low r er rim of the polariser or chromatic condenser by means of three 

 converging or equidistant clips situated below the stationary disk L S. 

 One of these clips may be displaced and fixed by a screw S. The dia- 

 meter of the first hole corresponds to that of the polariser tube ; from 



