CUTTING HARD SECTIONS 511 



soon as the finger itself begins to come into contact with the stone, 

 it must be guarded by a Hat slice of cork, or by apiece of gutta-percha 

 a little larger than the object. Under either of these, the section 

 may be rubbed down to the desired thinness ; but even the most 

 careful working on the finest-grained stone will leave its surface 

 covered with scratches, which not only detract from its appearance, 

 but prevent the details of its internal .structure from being as readily 

 made out as they can be in a polished section. This polish may be 

 imparted by rubbing the section with putty-powder (peroxide of tin) 

 and water upon a leather strap made by covering the surface of a 

 board with buff leather, having three or four thicknesses of cloth, 

 flannel, or soft leather beneath it ; this operation must be performed 

 011 both sides of the section, until all the marks of the scratches left 

 by the stone shall have been rubbed out, when the specimen will be 

 fit for mounting ' dry,' after having been carefully cleansed from any 

 adhering particles of putty-powder. 



Greater facility in the grinding of hard sections. as well as supe- 

 riority of result, is attainable by simple mechanical means. 



A cutting machine will greatly facilitate the process of preparing 



FIG. 411. Hand machine for cutting hard sections. 



rock slices. The thickness of each slice must be mainly regulated 

 by the nature of the rock, the rule being to make it as thin as can 

 be conveniently cut, so as to save labour in grinding down afterwards. 

 Perhaps the thickness of a shilling may be taken as a fair average. 

 This thickness may be still further reduced by cutting and polishing 

 a face of the specimen, cementing that on glass, and then cutting as 

 close as possible to the cemented surface. The thin slice thus left 

 on the glass can then be ground down with comparative ease. 



The first (fig. 411) is a hand machine. The specimen is cemented 

 to the carrier, a, which is movable on the axis, b, and can also be 

 rotated in two directions. The object is pressed by the weight, c, 

 against the steel disc, fZ, which i:? revolved by the wheel, e, acting on 

 a smaller-toothed wheel on the axis of d. 



The second (fig. 412) is intended to be worked by the foot. The 



parts a, b, c, and d are the same as before. The wheel and treadle at 



y"and g work the pulley, e, by which the steel disc, d, is revolved ; h 



is part of the cover for the disc, tc prevent the emery flying about. 



A box beneath also catches the powder that falls. 



(This arrangement is also supplied with fig. 411, though not shown 

 in the woodcut.) A second wheel at -i, with a cord passing over k, 



