146 THE AMERICAN MONTHLY [April 



one screw at the back near the bottom, passing through a slot 

 in the sliell-tube. Surrounding this is a Hglit wire spiral spring 

 l)ressing on the screwhead and against a fixed stud above it. 

 In front of the fine-motion tube is screwed an angle-piece pro- 

 jecting out and bearing on the point of the fine-motion screw. 

 We have tlms a down-pressure at the back of the tube and a 

 bearing in front. All this tends to keep the fine-motion tube 

 up to its bed at opposite places, front and back, so that no 

 amount of wear can ever cause the arrangement to get slack. 



The steel screw of 80 threads to the inch is carried by a staple. 

 It is tapped into the lower arm, but the upper blank end passes 

 through the top. Its pointed end bears onto a hard steel flat 

 let into the fine-motion arm. The milled head within the 

 staple is merely screwed hard on to the fine thread. Where the 

 inner tube bears it may be very slightly east^d off at the middle 

 to avoid the possibility of rocking sideways ; this does not occur. 

 The motion is perfect, and sensitive to the slightest touch. 



I now refer to the stage. As this may be considered the most 

 important feature, I append separate illustrations. However 

 well some may say that they can mmage to move an object- 

 slide about with the fingers, to the majority of us this is a tan- 

 talising and clumsy operation, and nearl}' everyone must ap- 

 preciate the luxury of a mechanical stage with rectangular 

 movements. In cheap microscopes this is prohibitive, on the 

 score of expense. Some of the old stages, with their rectilinear 

 slides, set-screws, and adjusting slips, are more appropriate for 

 a lathe slide-rest than for a mere carrier for a weight of a frac- 

 tion of an ounce. Fig. 2 is a plan of the stage half the size of 

 the original. It consists merely of a plane rectangular base- 

 plate, with the top perfectly fl.at, and perforated with two 

 holes. One embraces the pillar; the other is ledged for carry- 

 ing the diaphragm plate, or subtage illuminators, &c. The top 

 or moving plate (shown shaded) has two horns extending back, 

 allowing room between for sufficient range clear of the pillar. 

 Close to the outer edge of these horns are screwed two pieces of 

 fine rack, 1 3-8in. long. The final screwing down of these must 

 be deferred till the pinion is set in its bearings, in order to set 

 the top plate in exact parallelism with the lower one. Having 

 thus two racks spread some distance apart, each actuated by 

 the same pinion, a perfectly straight movement is obtained fore 



