126 Transactions. 



in tlie "English Meclianic," Yol. 10, pp. 208, 330 and 357. The diameter of 

 the reflecting speculum is 6^ in., and the focal length 4 ft. 9| in. The total 

 length of the tube, which is made of galvanized iron, being 5 ft. 9 in., and its 

 diameter Q\ 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. 



(5.) 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 : — 1. 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 



