Paksons. — On a Reflecting Telescope made in Wellington. 127 



Lord. Rosse for grinding his metallic speculum. Flower emery was used for 

 fine-grinding, made into the thickness of cream with water, and spread evenly 

 over the surface of the squares into which the surface of the tool was divided. 

 The machine was then driven at about sixty revolutions per minute, the 

 emery that escaped being collected, washed, and precipitated. This process 

 was repeated four times, a finer degree of emery being obtained by allowing it 

 a longer time to settle in each instance ; the last time the emery being so fine 

 that it took twenty-four hours to settle from the water, which was then drawn 

 oflf by a glass syphon. When the fine-grinding was completed the surface of 

 the speculum looked like plate-glass covered with a milky film, no grain being 

 visible with a magnifying glass. 



(c.) Polishing. — To efiect this the iron tool was faced with pitch | in. 

 thick, divided into squares by grooves in a similar manner to the iron tool 

 itself. The soft surface of the pitch when warm was moulded by the glass 

 speculum so as to fit correctly, and then covered with rouge purified from grit 

 by washing and settling in the same manner as the emery. The polishing was 

 then done on the same machine as the fine grinding, and occupied six hours of 

 continuous work, so that no change in the form of the pitch surface should 

 take place through great alteration of temperature. The utmost care had to 

 be exercised to prevent any dirt settling on the tool during this part of the 

 process. 



(d.) Paraholizing. — The curve of the speculum produced by the above 

 process was such that on being mounted on the tube it gave imperfect 

 defirdtion. To correct this, and to give the surface the perfect curve, was the 

 most important and delicate part of the whole process, and formerly was 

 efiected only with great difficulty. The use of the test bar already alluded to, 

 which is the invention of Mr. Purkiss, enables it to be done with comparative 

 facility. The strip of cardboard was fixed with the curved wood on the face 

 of the speculum. A star was then viewed with the eye-piece out of focus, so 

 as to get an enlarged disc of light crossed by a black band caused by the test- 

 bar. The edges of this band were found to be curved instead of straight, and 

 all that was required to produce a proper curve was to modify the polishing 

 surface of the tool by repeated trials until it imparted such a form to the 

 speculum that the image of the test bar had parallel edges. This was done by 

 scraping down the squares of the pitch on the surface of the tool so as to make 

 them proportionally larger or smaller, at the centre or margin, as experience 

 required. By this simple process a correction of the curve amounting to only 

 one-millionth of an inch can be made. 



(e.) Silvering. — The speculum having been thus polished to a true curvature 

 could be used for observation in the same manner as a metallic speculum, but 

 there was, of course, a great loss of light owing to the transparency of the glass. 



