LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
517 
Mr. Edwards mentions that he had found, that, unless the pitch was of sufficient thick- 
ness, it would not preserve its figure ; he had observed the fact correctly : in assign- 
ing the cause he was evidently in error, as he attributed it tq the circumstance that 
the thin coat of pitch, as he supposed, acquired more heat from the friction than the 
thicker, while the reverse must have been the fact, as the thinner the imperfect con- 
ducting material, the quicker the metallic plate, to which it was attached, would have 
dissipated the heat, by radiation and conduction. 
Mr. Edwards’s specula were very small, and he found the thickness of half-a-crown 
sufficient. It was, however, evident, both as a deduction from all that had been 
written upon the subject, and from some experiments on small specula, that the ne- 
cessity of using a layer of pitch thicker in proportion to the size of the speculum was 
a great evil, and was alone sufficient to make it impossible to polish large surfaces as 
accurately as small ones. A consideration of the theory, which I have ventured to 
put forward, suggested a very obvious remedy. It was very evident that by grooving 
the layer of pitch, provision might be made for its lateral expansion, wherever required, 
without so great a thickness. The experiment was first tried by reducing the thick- 
ness of pitch one-half, and making furrows in it by means of a hot iron quite down to, 
the metallic plate ; the furrows were two inches apart, and there were two sets at 
right angles to each other, so that there were nowhere more than four square inches 
of pitchy surface in continuity. The result was, that the defining power of the spe- 
culum was immediately much improved. After many trials, however, a far better mode 
of effecting the same object suggested itself. The furrows were with difficulty kept 
everywhere open, and where there was a failure in this respect the old evil recurred, 
the polisher lost its figure ; moreover, it was not found practicable to reduce the 
thickness of pitch to a minimum, which was a great object, and there were other 
minor practical inconveniences. These defects were all remedied by dividing the iron 
disc itself instead of the pitch ; this could be done to any degree of minuteness re- 
quired, and the continuous pitchy surface so reduced that its thickness might be made 
a minimum, in fact, not greater than necessary to satisfy the condition of enduring the 
small amount of abrasion which takes place during the time required to complete the 
polishing. The improvement which immediately followed this simple device was far 
greater than could have been anticipated, and the divided three-feet speculum, after 
this change, defined better with a power of 1200 than it had done before with a power 
of 300. Several polishers were made on this construction, the arrangement and di- 
mensions of the grooves being somewhat different : that last used in polishing the two 
three-feet specula I think is the best, and a drawing of it has been annexed, figs. 8 anti 
9 : the circular grooves were turned with a slide rest and are three-eighths of an inch 
deep and one quarter wide, leaving bands of continuous surface one quarter of an 
inch wide; the grooves at right angles are about one inch and a quarter apart, one 
quarter of an inch wide, and half an inch deep ; they were cut with a small circular 
saw, under which the polisher was made to traverse on the bed of a large lathe. 
