APPENDIX. 343 



in the centre of the cross piece (d) connecting the sides of the 

 car, runs a vertical iron rod (e) supporting at its lower extremity 

 a cast iron cross piece with flat ends (/), which hangs transversely 

 to the direction of the track through the central opening in the base 

 of the car. The ends of this cross piece reach under the projecting 

 inner edges of the flat iron rails (bb) and are made to clamp against 

 their under surfaces by a nut with handles (g), screwing on the upper 

 part of the iron rod, and binding on an iron washer on the wooden 

 cross piece (a) through which the rod runs. The car can thus be 

 fixed upon the track at any distance from the microscope within ten 

 feet, and the distance that the surface of the negative is from the 

 stage of the microscope in any given position is determined by a 

 scale of feet laid off upon the floor close to one of the rails, and a 

 scale of inches on the side of the base of the car (see fig. 417). 



" To obtain the final focus of the image upon the plate in the plate 

 holder, the following contrivance is used (see fig. 416). A perfectly 

 straight cylindrical iron shaft (a), runs the entire length of the track, 

 midway between the two rails, and at such a height as just to clear a 

 groove on the under surface of the base of the car. This shaft has a 

 shallow square groove cut in it along its entire length, and is supported 

 at each extremity by brass bearings attached to the floor, in which it 

 turns freely. To the posterior cross piece of the base of the car is fas- 

 tened a bent brass bearing (Z>), projecting into the square opening 

 in the base of the car and supporting two bevel gear wheels (<r) work- 

 ing into each other. The upper and horizontal one of these wheels 

 is turned by a vertical iron rod (d) attached to it, which is furnished 

 at its upper extremity with a large milled head (e) and is supported 

 by a collar (/) attached to the cross piece connecting the sides of the 

 car. The lower and vertical wheel is pierced to allow the passage of 

 the long shaft (a), and from the surface of the bore a small square iron 

 tongue projects, exactly fitting the longitudinal groove in the shaft. 

 By this means, no matter what may be the position of the car upon 

 the track, the operator can rotate the shaft (a) through the pressure of 

 this tongue upon the sides of the groove, by turning the milled head 

 (<?) connected with the bevel wheels. At the same time the car can 

 be moved freely over the track, the iron tongue running smoothly to 

 and fro in the grooves of the shaft. This long shaft (a) is made to 

 turn the fine adjustment wheel of the microscope by the follow- 

 ing means (see fig. 417). Attached to the edge of the shelf, upon 

 which the microscope stands is a short iron axle parallel to the 

 grooved shaft below, which turns freely in two flat brass bearings, 

 and supports two wheels. One of these, a small brass wheel, is 

 grooved and connected by a silk thread, removeable at pleasure, with 



