EXAMINATION OF STRUCTURE OF FOSSIL PLANTS. 15 



sists of a Watt's parallel motion, with four joints, attaclied 

 to a basement fixed to the table of the lathe. This base has 

 a motion (for adjustment only) in a horizontal plane, by 

 which we may be enabled to place the upper joint in a parallel 

 plane with the spindle of the lathe. This may be called the 

 azimuthal adjustment. The adjustment, which in an astro- 

 nomical instrument is called the plane of right ascension, is 

 given by a i)ivot in the top of the base, and clamped by a 

 screw below. This motion in right ascension gives us the 

 power of adjusting the perpendicular planes of motion, so 

 that the object to be slit passes down from the circumference 

 of the slitting-plate to nearly its centre, in a perfectly parallel 

 plane. When this adjustment is made accurately, and the 

 slitting-plate well primed and flat, a very thin and parallel 

 slice is obtained. This jointed frame is counteqioised and 

 supported by a lever, the centre of which is movable in 

 a pillar standing perpendicularly from the lathe table. 

 Attached to the lever is a screw of three threads, by which 

 the counterpoise weight is adjusted readily to the varying 

 weight of the object to be slit and the necessary pressure 

 required on the edge of the slitting-plate. 



The object is fixed to the machine by a pneumatic chuck. 

 It consists of an iron tube, which passes through an aperture 

 on the upper joint of the guiding-frame, into which is screwed 

 a round piece of gun-metal, slightly hollowed in the centre, 

 but flat towards the edge. This gun-metal disc is perforated 

 by a small hole communicating with the interior of the iron 

 tube. This aperture permits the air between the glass plate 

 and the chuck to be exhausted by a small air-syringe at the 

 other end. The face of this chuck is covered with a thin film 

 of soft india-i-ubber not vulcanised, also perforated with a 

 small central aperture. When the chuck is properly adjusted, 

 and the india-rubber carefully stretched over the face of the 

 gun-metal, one or two pulls of the syringe-piston is quite 



