ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 845 



remains unaltered in position. A plate of quartz for circular 

 polarization 3 ■ 75 mm. thick, and mounted in a little brass fitting, is 

 shown at z. It slides into a slot t, situated close to the lower end of 

 the inner microscope -tube and above the objective. The movement 

 imparted to the microscope-tube by the screws n m, tends to throw 

 the analyzer slightly out of position with regard to the polarizer, but 

 Professor Rosenbusch finds that this produces scarcely any appreciable 

 error. 



For very strongly convergent light, the ordinary condensing lens 

 (with a focus of 12 mm.) attached to the Nicol, is combined with a 

 second one of only 8 mm. The axes to these mineral sections can 

 thus be recognized without an eye-piece and with the objective 

 alone.* 



The stauroscopic eye-piece is shown in the side diagrams A and 

 B of Fig. 150. The eye-lens is attached to a separate tube sliding in 

 that holding the field-glass, and can be brought closer to the latter. 

 The tube of the field-glass has a slit in which a pin a, inserted in the 

 eye-lens tube, slides. The pin also passes into a slit in the micro- 

 scope-tube, and thus fixes the position of the eye-piece. At c is a plate 

 of calc-spar in the focus of the eye-lens. This was first used by 

 Professor Calderon for stauroscopic measurement, and afterwards 

 adapted to the Microscope by Fuess. For the purpose of accurately 

 indicating to the eye the correct position with regard to the optical 

 axis of the instrument, a cap with a very narrow diaphragm d is 

 added, there being another diaphragm at c. 



Professor Eosenbusch points out that the use of this Microscope is 

 not confined to the purpose for which it was designed, but that it is 

 also available for other microscopical purposes where exact measure- 

 ments are required.! 



(2) Fuess's " large Microscope for mineralogical and petrographical 

 observations" (Fig. 151), is designed as an improvement upon the pre- 

 ceding, especially as regards the stability of the centering arrange- 

 ment. The sliding coarse adjustment is done away with, and instead 

 of it, the stage c with the illuminating apparatus is attached to the 

 slide d which moves on the upright support / of the stand to a distance 

 of about 1*5 cm. (by means of the rack-work attached to d and 

 actuated by the pinion e), and is held in the required position by the 

 clamping screw behind. The fine adjustment is effected by the usual 

 graduated micrometer screw. By the use of intermediate pieces of 

 tubing the objectives can be so attached to a horizontal revolving 

 plate a, that they all stand at about their focal distance from the 

 object if the slide is of ordinary thickness. The centering of each 

 objective with the optic axis of the tube b is then effected by three 

 adjusting-screws below the revolving-plate, so that the arrangement 



* See this Journal, i. (1878) p. 207. 



t The graduation of the micrometer-screw and the addition of the plate of 

 calc-spar and the Wright's indicator, appears to have been suggested by Professor 

 A. v. Lasaulx. Cf. Bull. Soc. Belg. Micr., iv. (1878) p. clxxvi. The Microscope 

 described by M. Eenard, Bull. Soc. Belg. Micr., iv. (1878) cexv., and this 

 Journal, i. (1878) p. 270, appears to have been a Rosenbusch-Fuess instrument, 

 but with the Lasaulx improvements and the addition of the quartz plate. 

 Ser. 2.— Vol. II. 3 L 



