J.WUARV. 1911. 



KNOWLEDGE. 



31 



machine was first made this bearing was plain, and consisted 

 of a nut and washer behind the upright of the jaws and 

 another nut and washer in front, and these were simply 

 screwed together. This worked fairly satisfactorily, but stuck 

 somewhat when tightened sufficiently to pre\ent shaUe, and 

 the bicvcle pedal bearing was substituted. It will be seen 

 that the jaws, if properly adjusted, swing on the ball bearings 

 parallel to the cutting disc, and the rock can be mo\ed 

 forward for cutting another slice by turning the spindle 

 toward the operator. 



A disc of brass one and three-cjuarter inches in diameter is 

 fixed to the left hand end of the spindle as a handle for 

 turning it by, and this is graduated into four equal divisions 

 for reference. 



In order to get the rock-holder to swing strictly parallel 

 with the cutting disc, some adjustment is necessary, and this 

 is arranged by bolting the upright carrying the spindle to the 

 table. The bolt in front is a fi.xture, but the one in the rear 

 goes through an elongated hole in the table, so that the upright 

 can be swung on the front bolt as a pi\'ot until the jaws are 

 parallel, and then the back bolt is clamped up tight. 



The rock is kept against the cutting edge of the disc by 

 the chain and weight attached to the rock holder at the bottom 

 corner. This can be seen in the 

 illustrations. 



The grinding lap is a cast iron disc 

 eight inches in diameter, half an inch 

 thick in the centre, with the top siu'face 

 bevelled off to three-eighths of an inch 

 at the edge. A lead or copper lap can 

 be used instead of the cast iron one. 

 but I find the latter is quite satis- 

 factory and is cheaper to purchase. 



The spindle for this lap is made 

 from an ordinary half-inch bolt, si.x 

 inches long, procurable from any 

 ironmonger. This requires cutting 

 down until it is about four inches long, 

 then it has to be centred, and made 

 with a cone at the bottom end and a 

 cup at the top. A pulley wheel is 

 fixed at the bottom of this spindle to 

 take the driving band. Two nuts are 

 wanted for the top of this spindle, one 

 of which is screwed up tight for the 

 lap to rest on. and the other to fasten 

 it by. The bearing for this spindle 

 under the table is a screw with a cup 

 in the end, which is fixed with a couple 

 of nuts through a threaded plate. This 

 plate is attached to a piece of wood 

 affixed to the underside of the table. 

 The top bearing is a screw with a cone point which passes 

 through a threaded plate into the top of the spindle. This 



—ia 



Figure Z 



Figure 3. 



Figure 1. 



screw is adjustable for taking up wear and is securely fixed 

 by the lock nut. All this can be procured from a small cycle 

 maker at a very reasonable price. 



The carrier for this top bearing con- 

 sists of a block of wood seven inches 

 long by three inches high by two and 

 a half inches wide, and this is securely 

 screwed to the t.able from underneath. 

 Upon this is a piece of oak three 

 quarters of an inch thick, seven inches 

 long by three inches wide, to which is 

 screwed the threaded plate for the 

 coned screw of the bearing. 



This method of fixing has been 

 found quite satisfactory. It is per- 

 fectly rigid, and there is no shaking or 

 jumping of the lap, even when driven 

 at the highest speed obtainable. 



It will be seen that the drive for the 

 slitting disc is taken direct from the 

 (hiving wheel, but that for the grind- 

 ing lap goes over two jockey pulleys 

 so as to get the horizontal move- 

 ment. These jockey pulleys are 

 ordinary iron pulley wheels \\ ith 

 screw ends, such as are used for 

 cirrying blind cords. 



It is necessary to ha\e two leather 

 or gut bands, and these have to be 

 changed when altering from the slit- 

 ting disc to the grinding lap. 

 A piece of tin. shaped like a bicycle mud guard, is fixed to 

 the table behind the slitting disc to stop the splashes. A tin 

 tray is also placed below the table under the disc to catch the 

 waste carborundum and water, which can be used o\er again. 

 Round the grinding lap I use a cake tin, which has had a large 

 hole made in the bottom for the spindle and pulley wheel to 

 pass through. A ledge of tin is soldered all round this hole to 

 keep the water, and so forth, from the bottom bearing. 

 These mud guards are not shown in the photographs. 



This apparatus has been in use now for about three years, 

 and has ground between four hundred and four hundred and 

 fifty slides during that time. It has given the greatest 

 satisfaction to me. and although, no doubt, it is not so good as 

 a professionally made machine (with which, of course, it is 

 not intended to compete), it will be found a very useful article 

 for the amateur petrologist. who usually has far more 

 enthusiasm than he has money. p tt r^ppyN 



KCJVAL MICROSCOPICAL SOC I FTV.— November 16th, 

 1910, Professor J. Arthur Thomson, M.A., F.R.S.E., President, 

 in the chair. In connection with the gift of two slides of 

 Plcicrosigina. the Hon. T. Kirkman. the donor, asks whether 

 fresh-water Pleurosigmae with oblique striae are rare. 



