LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
519 
in contact with the speculum can thus be made as hard as necessary, while the thin 
subjacent layer of softer resin expands laterally, so as to preserve the figure of the 
polisher. The process of polishing for optical purposes appears to me in some mea- 
sure to resemble that of filing, the polishing powder imbedded in the resinous surface 
representing the teeth of the file : while the polisher preserves its figure exactly, and 
consequently its contact with the speculum is exceedingly close, every particle of 
polishing powder, as it insinuates itself between the rubbing surfaces, must be instantly 
forced into the resin, deeper as the resin is softer, producing a grooving or grain in 
the speculum, finer, if the fineness of the polishing powder is given, in proportion to 
the softness of the resin, and, consequently, to the depth the particles have become 
imbedded, and, therefore, the smallness of the portion of each which projects; but 
the moment the figure of the polisher ceases to be exact, then the polishing powder 
is no longer forced into the resin, but runs loose, producing a grain perhaps as coarse, 
or coarser than the size of the particles of the powder itself. Hence, therefore, in 
practice it is of no less importance to the production of a fine polish, than it is to the 
production of a fine figure, that the polisher should very exactly fit the speculum 
during the whole operation. I find invariably that the moment that exact coincidence 
ceases, the polish rapidly declines, and is soon completely spoiled. I have hitherto 
observed that the quality of the polish which yields the maximum of defining power 
is that which is technically called a black polish, provided a very fine grain is per- 
ceptible when the speculum is placed near a window. A speculum may be polished 
so that the surface appears black, and without grain, like a surface of quicksilver ; 
but I have always found it necessary for that purpose, to employ a softer resinous com- 
position than seems consistent with the production of a very true surface. Concei- 
ving that such a polish, though I did not find it reflected more light, was likely to re- 
flect more accurately, I tried a vast number of experiments with the view of obtaining 
it in conjunction with the truest surface, but hitherto without success : the subject, 
however, perhaps, deserves to be pursued further, and as it seems impracticable with- 
out injury to soften the resinous surface, the best chance seems to be to search for 
some polishing substance consisting of smaller particles than the fine peroxide of 
iron, the one I have always used, so as to produce a grain not exceeding the magni- 
tude which theory has assigned as that of an undulation of light. In preparing 
the resinous surface for the polisher, I have for a long time employed a mixture of 
common resin and turpentine, instead of pitch, having previously experienced much 
inconvenience in polishing large surfaces from the gritty particles which the pitch I 
was in the habit of using very frequently contained. However, whether pitch or 
resin be made use of, it is absolutely necessary that the hardness should be adjusted 
to the proper standard with great care. 
Mr. Mudge and Mr. Edwards have given different directions on this subject ; 
Mr. Mudge recommending the pitch to be rather soft, and Mr. Edwards very hard : 
but in the common mode of conducting the operation, no precaution being used to 
