F. E. Wright — New Petrographic Microscope. 411 



(1) An Abbe condenser is used, and with it a large nicol 

 prism, or an Alirens prism, 15 mm edge, after the manner of the 

 Fuess microscope No. la. With this arrangement the entire con- 

 denser lens system remains in position and its upper lens need 

 not be removed when low-power objectives are used. This does 

 away with the devices which have been employed for throw- 

 ing the upper part of the condenser combination out of the 

 axis of the optic system and which complicate the construction 

 considerably. 



(5) The selenite, or quartz, plate of sensitive tint is inserted in 

 a metal case at Q (lig. 1), just below the condenser. It is revolv- 

 able in the carriage F about the optic axis of the microscope, an 

 arrangement which often facilitates the determination of the 

 ellipsoidal axis of a particular section because the abrupt rise 

 or fall of interference colors on insertion and rapid revolution 

 of the plate appears more clearly than if the slower moving 

 stage itself were revolved. At M a combination wedge is 

 introduced as in ordinary microscopes. 



(6) The Bertrand lens E, fig. 1, is mounted on a sliding 

 arrangement which, in connection with the sliding ocular tube, 

 permits of different magnifications of the interference figure, 

 an arrangement already adopted on several well-known micro- 

 scopes. In the present microscope the focal length of the 

 Bertrand lens (55 mm ) has been so calculated that the initial 

 magnification of the interference figures can be varied from 

 •81 diameters to 1*90 diameters. The ocular itself magnifies 

 this image in turn eight-fold, so that the resulting magnifica- 

 tions range from about 6*5 to 15*2 diameters. The fact that 

 the upper nicol intervenes between the objective and Bertrand 

 lens limits very materially the range of magnifications possible 

 by the Bertrand lens. An iris diaphragm is introduced directly 

 below the Bertrand lens and slides up and down simultane- 

 ously with it. This diaphragm is opened and closed by means 

 of the pin, Y, fig. la, which is connected with the diaphragm 

 itself by means of pin and ratchet movement. 



(7) A second iris diaphragm is introduced at Gr, fig. 1, directly 

 below the ocular, and is used in connection with the observa- 

 tion of interference figures by the Lasaulx method without 

 the Bertrand lens. To be of service in this connection, the 

 iris diaphragm should be located precisely in the image plane 

 from the objective, as was emphazised especially by Czapski 

 in 1891,* for in that plane alone can light be excluded from 

 adjacent minerals in the thin section. To realize satisfactorily 

 this condition, the writer has heretofore used the cap stop 

 indicated by fig. 4, with two sets of slides, S, and S 2 , at right 



*Neues Jahrbuch, Beilage Band vii, 506, 1891. 



