SPECULA OE REFLECTING TELESCOPES. 
307 
vature of the speculum. The polisher is then to be painted with the best thin oil-paint, 
the process being continued or renewed until all the pores of the wood are perfectly 
saturated with the oil. When perfectly dry it is to be well varnished, and then will be 
ready to receive the pitch. For covering the polisher symmetrically with squares of 
pitch with due interstices (a most essential condition), I have used a peculiar apparatus, 
which really converts this troublesome and very unmanageable process into one not at 
all irksome and also cleanly, speedy, and efficient. This, which may be called a pitch- 
mould, is represented in Plate 52. fig. 9, half the real size, a b is a square prism of white 
deal, on the upper part of every side of which is hinged a piece of deal fitting closely to 
the prism, and extending about four tenths of an inch beyond the upper end of the prism. 
The outsides of the upper ends of these pieces are a little tapered, so that, when their 
sides or surfaces are in contact with the prism, as in the figure, a light zinc hoop ( e ) 
may be dropped over them to hold them in position. A cell or mould is thus made on 
the top of the prism about inch square and four tenths of an inch deep. The lower 
part of the prism (c d) is encompassed by a hoop of sheet lead, sufficient to make it sink in 
water and keep upright. To have five or six of these moulds saves time in the process 
of casting the pitch. Previous to being used, the moulds should be immersed for days, 
or at least 24 hours, in order that the pores of the wood may be so saturated with 
water that the melted pitch will not attach itself to any part of it. And to bear this 
treatment well, the pins, as well as the leaves of the hinges, should be brass, and the 
attaching screws also brass. Fig. 10 represents a cylindrical vessel of thin copper, 
about 11 inches in diameter and 11 inches deep, with a short copper tube hard- 
soldered in its side near the bottom, six tenths of an inch wide. This tube is to be 
fitted with a very long and slowly tapering mahogany plug, so as to give plenty of 
latitude for' regulating the flow of pitch. I use black pitch, made from Swedish or 
Bussian tar, and have obtained it of very good quality from Tolhurst and Sons, 
60 Tooley Street, in small kegs. Formerly I used to strain the pitch through muslin 
(a most disagreeable operation), but for many years I have forborne to do so, deeming 
it quite unnecessary; and some other niceties, such as polishing the speculum in 
water of the same temperature as the laboratory, also I have laid aside. The 
general tendency of my experience has been to approximate to the utmost simplicity 
consistent with accuracy of workmanship. The best way of opening a barrel of pitch 
is to saw the staves through in the middle all round, when by a smart blow or two it 
will generally break in the middle into two portions. By inverting one of them over a 
large sheet of brown paper and slightly tapping the loosened staves, any required 
quantity may be readily detached. The pitch is generally so hard that it will bear this 
treatment even in hot weather. The pitch-vessel may be conveniently placed on a low 
iron tripod, and the pitch melted by a Bunsen burner mounted at the end of a gas- 
bracket. The pitch is adjusted to the proper temperament by adding tar if it be too 
hard, and resin if too soft. If the latter has to be added, it should be melted in another 
vessel and poured in while fluid. The due attempering of the pitch may be secured by 
