52<5 Method of grinding and giving the true Figure 



and not of its genuine whitenefs. Pitch ma^ be eafily made harder by adding a proper 

 quantity of roCn. I often ufe equal quantities of pitch and rofin, fo as to make the mixture 

 juft fo hard, when cold, as to receive an impreffion from a moderate prefTure of my nail^ 

 A poliflicr made with pitch and rofin has this advantage, viz. though it is hard, yet it is not 

 fo brittle as when pitch only is ufed, and made hard by boiling it, and confequently not fo 

 liable to break or chip off at the edges, and thereby fcratch the metal. Pour the melted 

 pitch and rofin when pretty cool from the crucible upon the elliptic tool,* fo as to cover it 

 every where when fpread upon it with an iron fpatula, about the thicknefs of a half-crown 

 piece. If the covering is too thin, it will continually alter its figure by the heat it acquires 

 in working the metal upon it, and thereby give a bad figure alfo to the fpeculura. When 

 it is fomewhat cool, lay a piece of writing paper upon the furface of the j>itch, pref^ 

 the face of the mirror upon it, and you wilf find the polifher will be nearly figured 

 to the form of the fpeculum. If it has not taken an exadl figure every where^ 

 which would appear by the fine marks of the grain of the paper upon the pitch, gently . 

 warm the furface of the pitch, and repeat the operation as before, until you have formed 

 it of the exa£l t figure of the metal. With a penknife "take away now all the fu- 

 perfluous pitch from the edge of the polifher, and with a conical piece of wood form 

 the hole in the middle accurately round : in other words, let the pitchy furface be every 

 where of the exaft Cze and fhape as tlie lead tool, which is under it. It may be 

 neceflary to mention, that the hole in the middle of the poliflier fhould go quite through 

 the tool, (for a particular reafon) and fhould be made of the fame fize, or fome-^ 

 what lefs, than the hole in the middle of the fpeculum. This is a neceflary caution, and 

 indeed I have always found that fmall mirrors, without any hole in the middle, will polifh. , 

 much better, and the figure will be more correal, if the polifher has a hole in the middle ' 

 of it. The powder I prefer above all others, to give a mofl exquifite luflre, is colcothcw of 

 vitriol^ and not putty. Putty gives metals a white luflre, or as workmen call it, a filver 

 hue ; but good colcothar of vitriol will polifh with a very fine and high black luflre, fo as 

 to give the metal finifhed with it the complexion of poliflied fleel. To know if the colco- 

 thar of vitriol is good, put fome of it into your mouth, and if you find it difToIves away 

 it is good ; but if you find it hard, and crunch between your teeth, then it is bad, and not 

 well burned. Good colcothar of vitriol is of a deep red, or a deep purple colour, and is 

 foft and oily when rubbed between the fingers ; bad colcothar of vitriol is of a light red 

 colour, and feels harfli and gritty. The colcothar of vitriol fhould be levigated between 

 two furfaces of poliflied fleel, and wrought with a little water; when It is v/orked dry,, 



• Thft elliptic tool muft be made pretty warm, or the pitch will not adhere to it. 



t When the poliflier is brought to its true figure, gently warm it at the fire, and with the edge of a 

 knife divide it into fevecal fquares, by pieffing the edge of the knife gently upon the pitch ; thefe fquares, 

 by receiving the fmall portion of the metal that works off it in polifliing, will caufe the figure of the fpe- 

 culum to be more concft, than if no fuch fquarcs had been made. The poliflier may alfo be formed with- 

 out the writing paper, by dipping the mirror into cold water, and aftsrwards prefling it upon the furface 

 of the pitch, (when it is fomewhat cool) and by repeating this operation till it has taken the exaft figure 

 of the metal. 



4 y°ii 



