MICROSCOPY. 



503 



tion.* Substitutes for the rack (such as a chain 

 movement, a coarse screw geared with toothed 



pinion is located high up upon the limb, to though not, in skillful hands, less precise than 

 give enlarged range for low powers. The the rack itself. 



movement is upon plane surfaces back of the The fine adjustment is by means of a screw 

 body, the racking of the round body through at the back of the limb, moving the whole 

 a hollow cylinder having been abandoned in body, usually by means of an intervening lever. 

 all good work on account of its imperfect ac- This arrangement is now practically universal, 



having replaced the screw with short lever in 

 front of the body acting upon a movable nose- 

 piece, which was the prevalent form in Ameri- 

 can and English stands only a few years ago. 

 In Mr. Zentmayer's model, introduced at the 

 Centennial Exhibition, the motion is upon 

 plane surfaces just behind those of the coarse 

 adjustment; in Mr. Bulloch's best model the 

 pinion-box of the coarse adjustment itself is 

 moved and with it the whole body ; in the 

 Bausch & Lornb stands a frictionless motion, 

 not subject to wear, and incapable of lost mo- 

 tion, is obtained by supporting the body by 

 two parallel horizontal steel springs a a, Fig. 

 9, while it is carried downward against the 

 reaction of the springs by means of the screw 

 and milled head 5 acting upon the pinion-box/ 

 by means of the inflexible bar e;* while Mr. 

 E. Gundlach adopts the 

 differential screw as a 

 means of securing a slow 

 or rapid motion at will. 

 In a recent ingenious 

 adjustment by Swift & 

 Son, of London, the fo- 

 cusing screw is placed at 

 the side of the limb, and 

 raises the objective by a 

 wedge movement, as 

 shown in Fig. 10; the 

 arrangement being giv- 

 en any required degree 

 of delicacy by varying 

 the pitch of the wedge. 



The stage is round, 

 thin, and concentric to 

 the optical axis of the in- 

 strument. This style, in- 

 troduced mainly through 

 the influence of the late 

 R. B. Tolles, of Boston, 

 is now generally recog- 

 nized and imitated as a 

 material improvement 

 on other forms. A plain 

 circular plate, as used in 

 Fig. 8, becomes the basis 

 wheels to the large milled heads, a diagonal for a movable upper plate, with or without a slid- 

 slot, or side-bars with crank-pin attachments) ing object-carrier, forming an excellent revolv- 

 have been abandoned or confined to individ- ing stage as shown (with object-carrier) lying at 

 ual makers ; though sliding the body by hand foot of same stand, and (with plain spring clips) 

 within an outer tube is retained in the cheaper in situ on the " arc " stand, or for an elaborate 

 stands as a simple arrangement, less convenient, mechanical stage with pinions both on one axis, 



FIG. 8. HISTOLOGICAL STAND : op THE AMERICAN TYPE. 



* A diagonal rack and spiral pinion is thought to give a 

 smoother movement, on account of its greater surface of con- 

 tact and longer hold. It was applied to the microscope as 

 long ago as the time of Pritchard, and is now revived by Ross 

 and Swift, of London; the latter making it sometimes double 

 or treble, at the suggestion of Mr. J. Mayall, with the teeth of 

 each part set out of line with the other, in order to give the 

 smoothness of a fine with the strength of a coarse pitch (" J. 

 B. M. 8.," 1884, p. 958). 



and with its mechanism wholly within the limits 

 of the circle so as to admit complete rotation, as 

 devised by Mr. Tolles, and adopted with more 

 or less changes by Mr. Bulloch, in his "Con- 



* This adjustment has since been adopted by Mr. Henry 

 Crouch, of London. In the German stands of Seibert & 

 Krafft a somewhat similar motion is attained by means of 

 hinged, parallel, horizontal bars. 



