Parsons.—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 
off 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 effect this the iron tool was faced with pitch 3 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.) Parabolizing.—The curve of the speculum produced by the above 
process was such that on being mounted on the tube it gave imperfect 
definition. 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 
effected 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.) Stlvering.—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. 
