170 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



yet whenever I chanced to find that a metal, when first applied to the polisher, 

 took the polish equally all over, and consequently the whole business did not 

 take up above 10 minutes; under those circumstances, I always used to please 

 myself with the expectation of a correct figure, at least as much so as the metal 

 had received from the hones, where the surface was but just and equally taken 

 off by the putty; but in this I constantly found rnyself deceived, and the metal 

 turned out good for nothing. In short, at this time, though I speculatively 

 knew that a parabolic figure was necessary to a perfect image, I yet considered 

 it as of little practical consequence. 



From the foregoing experiments, and a number of succeeding trials, I at 

 length discovered a certain way of giving a correct parabolic figure, and an 

 exquisite polish at the same time. This, which I have strong reasons to believe 

 was Mr. Short's method, I will now describe in as few words as I can. 



How to polish the speculum. — It is first necessary to observe that, in order to 

 avoid the detrimental intrusions of any particles of emery, it would not be right 

 to polish in the same room where the metal and tools were ground, nor in the 

 same clothes which were worn in the former process ; at least it would be neces- 

 sary to keep the bench quite wet, to prevent any dust from rising. Having then 

 made the polisher, by coating the brass convex tool equally with pitch, which we 

 suppose smoothed and finished with the brass tool in the manner before descri- 

 bed, and which is a very easy process, the whole operation is begun and finished 

 in the following manner. The leaden weight or handle on the back of the 

 metal should be divided into 8 parts, by so many deep strokes of a graver on the 

 upper surface of the lead, marking each stroke with the numbers 1, '2, 3, 4, and 

 so on, that the turns of the metal in the hand may be known to be uniform and 

 regular. To prevent any mischief from coarse particles of putty, I always wash 

 it immediately before using. In order to this, put about half an ounce of putty 

 into an ounce phial, and fill it ■§- with water ; then having shaken the whole, let 

 the putty subside, and stop the bottle with a cork. lu a tea-cup with a little 

 water, there should be a full-sized camel's hair brush, and a piece of dry clean 

 soap in a galley-pot : a soft piece of sponge will also be necessary. These, as 

 well as the metal bruiser and polisher, should be constantly covered from dust. 



The polisher being fixed down, and the camel's hair brush, being first wetted 

 and rubbed a little over the soap, let every part of the tool be brushed over with 

 it ; then work the bruiser with short, straight, and round strokes, lightly on 

 the tool, and continue to do so, now and then turning it, till the polisher have 

 a good face, and be fit for the metal. Then having shaken up the putty in the 

 phial, and touched the polisher in 5 or 6 places with the cork wetted with that 

 and the water, place the bruiser on the tool, and give a few strokes on the 

 putty, to rub down any gritty particles ; after which, having removed it, work 



