12% Proceedings of the British Association. 



powder employed runs loose between two hard surfaces, and may 

 produce scratches possibly equal in depth to the size of the particles : 

 in the polishing process the case is very different ; there the particles 

 of the powder lodge in the comparatively soft material of which the 

 surface of the polishing tool is formed, and as the portions projecting 

 may bear a very small proportion to the size of the particles them- 

 selves, the scratches necessarily will be diminished in the same pro- 

 portion. The particles are forced thus to imbed themselves, in con- 

 sequence of the extreme accuracy of contact between the surface 

 of the polisher and the speculum. But as soon as this accurate 

 contact ceases, the polishing process becomes but fine grinding. It 

 is absolutely necessary, therefore, to secure this accuracy of contact 

 during the whole process. If the surface of a polisher, of consider- 

 able dimensions, is covered with a thin coat of pitch, of sufficient 

 hardness to polish a true surface, however accurately it may fit the 

 speculum, it will very soon cease to do so, and the operation will fail. 

 The reason is this, that particles of the polishing powder and abraded 

 matter will collect in one place more than another, and as the pitch 

 is not elastic, close contact throughout the surfaces will cease. By 

 employing a coat of pitch, thicker in proportion as the diameter of 

 the speculum is greater, there will be room for lateral expansion, and 

 the prominence can therefore subside, and accurate contact still con- 

 tinue ; however, accuracy of figure is thus, to a considerable extent, 

 sacrificed. By thoroughly grooving a surface of pitch, provision may 

 be made for lateral expansion contiguous to the spot where the undue 

 collection of polishing powder may have taken place. But, in prac- 

 tice such grooves are inconvenient, being constantly liable to fill up : 

 this evil is entirely obviated by grooving the polisher itself, and the 

 smaller the portions of continuous surface, the thinner may be the 

 stratum of pitch. 



" There is another condition, which is also important, that the 

 pitchy surface should be so hard as not to yield and abrade the softer 

 portions of the metal faster than the harder. When the pitchy sur- 

 face is unduly soft, this defect is carried so far that even the struc- 

 ture of the metal is made apparent. While, therefore, it is essential 

 that the surface in contact with the speculum should be as hard as 

 possible, consistent with its retaining the polishing powder, it is 



