414 F. E. Wright — New PetrograpAic Microscope. 



was used over thirty years ago by Dick in London, but was 

 applied only to the revolution of a cap nicol above the ocular 

 in conjunction with the polarizer. It was, therefore, slightly 

 different from the present disposition. (2) The upper nicol 

 always remains in the tube and the substage nicol is inserted 

 or withdrawn on passing from ordinary light to crossed nicols. 

 This device eliminates the annoying change of focus and shift 

 of field ordinarily experienced when the nicol is inserted in 

 the upper tube. (3) The sensitive plate is inserted just below 

 the condenser and fits in a carrying device which is revolvable 

 about the axis of the optical system. This disposition has been 

 found convenient in determining the relative ellipsoidal axes 

 in a plate, since the plate can be revolved more quickly and 

 easily than the microscope stage or the two nicols together. 

 (4) It has a new mechanical stage of novel design and simple 

 but effective mechanical construction. The stage is practically 

 dust-proof, has a free upper plate and a movement of 24 mm in 

 any direction. (5) The Bertrand lens is fitted in a sliding 

 device such that the magnification of the interference figure 

 can be varied from 6*5 to 15*2 diameters. Immediately below 

 the Bertrand lens, the iris diaphragm is introduced, while 

 above this lens a second lens of short focal length can be 

 thrown into the field which, together with the ocular, serves 

 the purpose of focusing the image picture sharply in the 

 iris diaphragm. (6) The second iris diaphragm at Gr, immedi- 

 ately below the ocular, is intended for use when observing 

 interference figures directly by the Lasaulx method without 

 the ocular and Bertrand lens. This iris diaphragm is a substi- 

 tute for the device indicated in fig. 4, and the round cap 

 plates usually furnished with microscopes, and although theo- 

 retically less satisfactory, practically it answers the purpose 

 sufficiently well. (7) A large Abbe condenser is used, together 

 with an Ahrens prism of 15 mm edge, or large nicol in place of 

 the usual nicol and condenser with removable upper lens. 

 This arrangement, first introduced on the Fuess microscope la, 

 is a marked improvement over the usual arrangement and does 

 away with the more or less complicated devices for removing 

 the upper condenser lens from the optic axis of the microscope. 



