278 Transactions of the Society. 



On examining sections of material such as tin plate sheets, 

 similar supports of metal must again be provided. Fine cuts with 

 a fret saw are made in the centre, half way through the support, 

 and pieces of tin plate are slipped into the slits. They are retained 

 there by squeezing the metal in such a way as to cause the two 

 sides of it to press against the enclosed pieces of sheet. 



Polishing the Specimens. 



In general practice, it has been found that the polishing is more 

 rapidly effected by holding the specimen in the hand than by fixing 

 it in the automatic holders. One's finger tips suffer somewhat 

 when many samples are polished, but that is not a serious objection. 



Block No. 1 is placed on the wheel, and by means of a switch 

 the current of electricity is turned on to the motor. As the hori- 

 zontal wheel revolves, the specimen, sawn or filed to shape, is 

 pressed on to the surface of the block with considerable force at 

 first, and then with gradually diminishing pressure until the saw 

 or file marks are removed. This operation does not take more 

 than a minute. The current is switched off, No. 1 block is replaced 

 by No. 2, and the specimen passed over this. After changing 

 No. 2 for No. 3, the specimen is rough polished thereon. No. 4 

 block (cloth cover) is now put on the wheel, water is run on to the 

 surface, and the final polishing completed. 



One great secret of success in polishing is to gradually diminish 

 the pressure of the specimen on the blocks, commencing with heavy 

 pressure and finishing with practically none, and to pass the speci- 

 men round the block in an opposite direction to that in which the 

 wheel is rotating. 



The block with the " 000 " emery covering requires some little 

 preparation before it is suitable for fine grinding. This is effected 

 by covering it with rouge, and pressing a flat piece of polished steel 

 upon its surface when rapidly revolving. After rotating for about 

 five minutes, the surface of the block is rubbed with a piece of fine 

 linen cloth to remove all the gritty particles. This operation is 

 twice repeated, after which the surface will be in a most suitable 

 condition for practical work, and if care is taken will be capable of 

 polishing from fifty to sixty specimens. 



The cloth block, in spite of all precautions, may occasionally 

 become contaminated by dust and grit. This is very soon dis- 

 covered by the appearance of curved lines upon the specimen which 

 is being polished. In such case the block is removed and placed 

 in running water, and the cloth is meanwhile rubbed with the finger. 

 In this way the grit will become dislodged. 



h When polishing upon this block it is important that it should 



