126 Transactions. 
in the “English Mechanic,” Vol. 10, pp. 208, 330 and 357. The diameter of 
the reflecting speculum is 54 in., and the focal length 4 ft. 93 in. The total 
length of the tube, which is made of galvanized iron, being 5 ft. 9 in., and its 
diameter 64 in. The telescope is mounted on an equatorial counterpoise 
stand, with simple rack motion worked by hand, and on clear nights gives 
definition up to 300 diameters. The instrument was constructed in spare 
time, extending over eighteen months, but the real time employed if it was 
steadily worked at would occupy about six weeks. A second instrument, on 
the same principle, but with a 9 in. speculum, and 7 ft. focus, has also been 
constructed with the same machinery by Mr. J. Widdup, the speculum of 
which is also exhibited. The chief portion of the time was occupied in 
making the speculum, the remainder of the work being of an ordinary 
mechanical character. In making the speculum there are five distinct 
processes : rough-grinding, fine-grinding, polishing, parabolizing and silvering. 
(a.) The first step is to construct an iron tool having the required curva- 
ture, the model from which this is cast being made in the following manner :— 
With a radius of double the length of the proposed focus curved gauges or 
templates were cut in zinc, from which the wooden model is turned, and the 
casting made. The tool was then fixed firmly on an upright pillar, and 
covered with coarse emery powder. The glass for the speculum, which in this 
case was originally the side light of a steamer ? in. thick, was then fixed to a 
board and laid on the tool with a 28 lbs. weight on the top, and ground by a 
pulling and pushing motion round the pillar until it conformed to the test of 
the convex gauge. : 
(b.) Fine Grinding.—To effect this the surface of the convex iron tool was 
cut by cross grooves which divide its surface into squares, the object of which 
is to diffuse the fine emery evenly over the surface, and .so produce accuracy 
of figure. Before proceeding, however, the curve of the tool was perfected by 
what is termed file-testing. A few drops of coloured oil were first spread on 
the surface of the rough ground speculum, which was then laid gently on the 
tool so as to leave a mark on those portions which required filing—a delicate 
process, which had to be repeated over and over again till every part of the 
surface of the tool was equally marked with the oil. Before proceeding to 
fine-grind the speculum it was necessary to make what is termed the test-bar, 
to be used in an after process. This is a slip of cardboard, with parallel edges 
fixed on a rod, the surface of which is made accurately to fit the curve of the 
_ Speculum. To proceed with the fine-grinding the tool and speculum were 
mounted on a machine contrived so as to give four distinct motions :—l. A 
slow rotation to the grinding tool. 2. Direct back and forward stroke. 3., A 
transverse stroke. 4. Free rotation to the speculum on its own axis. This 
_ machine is a cheap, but quite serviceable, form of that which was designed by 
ma 
