504 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and mechanically simple in construction. In the figure the vertical edge or 

 rim of the cap of the stage-plate is indicated by the shaded broken 

 circle, the upper surface of this plate being considered removed, and the 

 working parts as seen from above thus exposed to view. The small 

 plates I a and I 2 are attached to the lower stage, and are so constructed 

 that the wedge-shaped edges allow the rectangular plate K to move only 

 in an east-west direction. This movement is effected by means of the 

 screw Ho. The plates J 1? J 2 , and J 3 of fig. 55, on the other hand, are 

 attached to the upper movable plate 0, and their wedge-shaped edges are 

 so adjusted that they allow the upper plate to move only in a north- 

 south direction with reference to the rectangular piece K. The screw H, 

 which terminates in a block attached to the upper plate, and running in 

 a sliding-pin I), accomplishes these north-south movements. The heads 

 of both screws B 1 and H 2 have divisions reading to O'Ol mm. move- 

 ment. Springs, not indicated in the figure, have been introduced, and 

 oppose the forward motion of the screws Hj and H 2 , and thus obviate 

 errors due to lost motion in the screws. The total movement of the 

 stage-plate in any direction is 24 mm. Mechanically, it is of simple 

 construction, aud consists of few parts. 



3. The metal part, containing iris-diaphragm and polarizer, can be 

 withdrawn from the optic axis of the Microscope by means of a release 

 spring, not shown in fig. 54. This part is also revolvable by itself about 

 the axis. This arrangement was adopted in preference to the usual 

 method of inserting and withdrawing the upper nicol, because of the 

 disturbing effect which the introduction of the upper nicol causes, both 

 on the focus and position of the field. With the present disposition, the 

 upper nicol remains permanently in the upper tube, and the optical 

 system — objective, nicol, and ocular— is not disturbed in passing from 

 ordinary to polarized light. In certain Microscopes the effect of the 

 upper nicol on change of focus is compensated by means of a small lens 

 of weak magnification, but even after the introduction of this device 

 some shifting of the field may still be experienced on inserting the upper 

 nicol. 



4. An Abbe condenser is used, and with it a large nicol prism, or an 

 Ahrens prism, 1 5-mm. edge, after the manner of the Fuess Microscope 

 No. 1a. With this arrangement the entire condenser-lens system re- 

 mains 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 throwing the upper part of the condeuser combination 

 out of the axis of the optic system, and which complicated the construc- 

 tion considerably. 



5. The selenite or cpiartz plate of sensitive tint is inserted in a 

 metal case at Q (fig. 54), just below the condenser. It is revolvable on 

 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 colours 

 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. 



G. The Bertrand lens E, fig. 54, is mounted on a sliding arrangement 



