FOE THE MICROSCOPE. 789 



joint in a parallel plane with the spindle of the lathe. This may be 

 called the azimuthal adjustment. The adjustment, which in an 

 astronomical instrument is called the plane of right ascension, is given 

 by a pivot in the top of the base, and clamped by a screw below. This 

 motion in right ascension gives us the power of adjusting the perpen- 

 dicular 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 counterpoised and supported by a 

 lever, the centre of which is movable in a pillar standiog perpendicu- 

 larly from the lathe table. Attached to the lever is a screw of three 

 threads, by which the counterpoise weight is adjusted readUy 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 communicat- 

 ing 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-rubber not vulcanised, also perforated with a 

 small central aperture. When the chuck is properly adjusted, and 

 the Indiarrubber carefully stretched over the face of the gun-metal, 

 one or two pulls of the syringe-piston is sufficient to maintain a very 

 large object under the action of the slitting-plate. By this method 

 no time is lost ; the adhesion is made instantaneously, and as quickly 

 broken by opening a small screw, to admit air between the glass plate 

 and the chuck, when the object is immediately released. Care must 

 be taken, in stretching the India-rubber over the face of the chuck, 

 to make it very equal in its distribution, and as thin as consistent 

 with strength. When this material is obtained from the shops, it 

 presents a series of slight grooves, and is rather hard for our pur- 

 pose. It ought, therefore, to be slightly heated, which renders it 

 soft and pliant, and in this state should now be stretched over the 

 chuck, and a piece of soft copper wire tied round it, a slight groove 

 being cut in the periphery of the chuck, to detain the wire in its place. 

 When by use the surface of the India-rubber becomes flat, smooth, 

 and free from the grooves which at first mar its usefulness, a specimen 

 may be slit of many square inches, without resort being had to 

 another exhaustion by the syringe. But when a large, hard, sili- 

 ceous object has to be slit, it is well for the sake of safety to try 

 the syringe piston, and observe if it returns forcibly to the bottom 



