ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 705 



is a convenience which has only to be once, used to be appreciated. It 

 not only allows the utmost freedom of movement, but also aids in the 

 maintenance of a true surface on the lap for a long time. The spindles 

 of the laps are somewhat shorter than that of the sheer, their length 

 being 12 in., so that the lap surface is about 3h in. above the table, 

 which is a convenient height for most operators. The mounting of the 

 spindles is the same throughout, and has already been described. Dust 

 and grit are excluded from the bearings by means of a special close- 

 fitting collar in each case. 



It will be seen that each grinding spindle is accompanied by a pillar 

 which supports a clamping device, in which specimens or blocks of glass 

 are held so as to swing radially across the laps. This permits of parallel 

 grinding to a precise thickness or definite form, and though not neces- 

 sary for ordinary rock slicing, it has, as already explained, a variety of 

 uses where precision is required. 



The lower portion of each rod is threaded, and screws into a long 

 socket let into the surface of the table. It can thus be accurately raised 

 and lowered during use, so as to maintain a steady and even pressure 

 upon the lap. It is also adapted to carry the goniometer, which fits the 

 special holder shown in the centre tray, and, as already indicated, as 

 every part of the machine is interchangeable if so required, a rapid 

 transfer from one lap to another can be made. This correspondence be- 

 tween the several parts of the machine, and the facility of interchange 

 which is thereby effected, results in the long run in a considerable saving 

 of time. 



The most effective laps, so far as my experience goes, are those of 

 bronze containing a high percentage of copper, the aim being to secure 

 a tough but not unduly hard lap. Pure copper laps would no doubt 

 be better, but they are difficult to cast and turn. Discs of lead and 

 tin, and also of wood with felted surfaces, are used in special cases and 

 for polishing. A diameter of 10 in. is found to be convenient for most 

 of these laps. 



Tray-like shields, or mud-guards, are provided for each of the 

 grinding laps, as well as for the sheer. As will be seen, they are of 

 square outline and conveniently large, the distance between the several 

 spindles, 22 in., permitting of this. The trays are made of stout gal- 

 vanized iron, 5 in. deep, and the upper edges are rounded and brass- 

 bound, forming clean and comfortable supports for the hands and 

 arms of the operator. It should also be noticed that a space around 

 each pillar or spindle is raised and carefully capped, so as to exclude 

 dust and grit ; this, in addition to the brass collars already noticed. 

 The bearings of a machine running at a high speed, and upon which car- 

 borundum and other abrasives are to be freely used, cannot be too care- 

 fully protected from their intrusion : the life of the bearings is, in fact, 

 directly proportional to the effective exclusion of the abrasives. 



As already stated, the machine is motor driven, and as the method of 

 connecting-up is in some respects novel, I refer to it in some detail. 

 An electrically driven 1 -horsepower motor serves to run the rock-slicer, 

 lathe, emery wheel, and polisher, and has proved fully adequate for all 

 requirements. As the motor runs at 1400 revolutions per minute, the 



