INVERTEBRATA, CRVPTOGAMIA, MIOUOSCOPY, ETC. 1009 



broad tripod base with revolving platform, bevelled, silvered, and gra- 

 duated in degrees for measuring the angular aperture of achromatic 

 objectives. Upon this platform are two pillars, between which the bar 

 and trunnions (which are of one piece) swing for inclining the instru- 

 ment to any angle. 



The coarse adjustment is effected by rack and pinion. The fine 

 adjustment (in all other instruments of the Jackson principle in front 

 of the body) is removed to the more stable part of the instrument, 

 the bar, which is provided with two slides, one for the rack-and-pinion 

 adjustment, and close to it, another one of nearly the same length, 

 for the fine adjustment, moved by a lever concealed in the bent arm 

 of the bar, and acted upon by a micrometer screw. In this way the 

 body is not touched directly when using the fine adjustment, and 

 the body does not change the relative distance of objective, bino- 

 cular prism, and eye-piece. (A woodcut of the fine adjustment will 

 be found at p. 321 of vol. ii.) 



The sioinging suhstage, which carries the achromatic condenser or 

 other ilhiminating apparatus and the mirror, swings ai'ound a pivot 

 placed behind the stage, of which the axis passes through the object 

 observed, so that the object is in every position in the focus of the 

 illumination. This most important arrangement, without which no 

 Microscope can be considered complete, is carried out in an extremely 

 simple and substantial manner. Although provided with but a single 

 joint, it admits of being swung over any of the stages ; a complete revo- 

 lution is only interfered with by the body of the instrument. It is 

 provided with a graduated circle at the upper collar for registering 

 the degree of obliquity, and a stop to indicate when it is central with 

 the main body. 



The substage is divided into two cylindrical receivers, to facilitate 

 the adaptation of several accessories at one and the same time. The 

 upper cylinder has centering adjustment, the lower cylinder of the two 

 can be moved up and down or entirely removed. 



As an object i)laced on the stage is in a plane with the axis of the 

 trunnions, it is obvious that, if the instrument is placed in a hori- 

 zontal position, the object is in the axis of revolution of the graduated 

 platform, and tlie angular a2)crturo of an objective focusscd on this 

 object can be easily measured. It can be readily seen that in tliis 

 position the obj(!ct is in tlie centre of all the revolving parts of tlic 

 instrument, the revolving stage, swinging substage, and the platform." 



There are three Stages: Ist. One devised by Mr. Zentmayer in 

 l.Sr.2 (sliown in position on the stand, Fig. 131), which is 5^ inches 

 in diuiiictcr and i inch in thickness. " It consists of a bell-metal ring, 

 firmly attached to the bar, l)ut ailjustalde by means of set screws, in 

 order to niukc it perfectly (•(tncentric to tli(> optical axis of the instru- 

 ment. This ring nceivcs the stag(^ platform, which has a complete 

 rcv(dution. Tlio outer edge is bevelled, silvered, and graduated into 

 degrees to serve as a goniometer. The carriage on whieli tlu^ object 

 is placed rests on a piece of plate glass, kej)t down by a spring with 

 an ivory-pointed screw to the two rails on the revolving stage plat- 

 forni, whicli gives an exceedingly smooth and firm niov<;nient, and a 



VOL. III. 1 B 



