692 PHILOSOPHICAL TRANSACTIONS. [aNNO I^OQ. 



wards for the polishing of the two surfaces of your object glass, and therefore 

 called the polishing tool. 



Prepare a piece of straw-coloured glass, of the plate glass kind, of the proper 

 diameter for the object glass desired, which ought always to be broader than the 

 proper aperture for that length ; let this piece of glass be ground flat, in another 

 tool, on both sides, and as nearly parallel as may be, and somewhat polished, in 

 order to discover whether there are any veins or flaws in the glass. When you 

 are satisfied of the goodness of the glass, you are then to prepare a handle to 

 fasten your glass to. Great care must be taken in this, for fear of bending the 

 glass by the handle: my method is this: I take a flat piece of brass, or rather 

 of the concavity of the sphere, to which the glass is to be ground; this piece of 

 brass should not be thicker than -f- of the thickness of the glass, of a circular 

 form, less in breadth somewhat than the glass itself, and having sides of the same 

 form, at right angles to the flat piece of brass, and these sides ought to be of 

 such a shape as that the fingers may easily apply to it in working, and these sides 

 should be as low as may conveniently be, and no thicker than about -| of the 

 glass. This handle is to be fastened to the glass, by warming the glass and 

 handle gently before a fire, and laying some pitch on the glass thus warmed, till 

 it becomes soft like melted wax ; and then laying the brass handle, a little 

 heated, on the pitch, you press it a little, till you are sure there is nothing be- 

 tween the glass and handle but pitch ; you then lay down the glass and handle on 

 something flat, taking care that the handle is in the middle of the glass, till it is 

 entirely cold. It is very material to know, that the pitch, to be used for fasten- 

 ing the handle to the glass, must be soft pitch, that has never been used nor 

 melted; for any other pitch will infallibly bend the glass. 



You then grind your glass in the concave tool with emery, and give it the 

 |irbper figure and smoothing for the last polish, in the common manner. 



In order to give the glass the last polish, which is the most difficult part of 

 the whole work, prepare some pitch for covering the before-mentioned polishing 

 concave tool, which is done in this manner: take some pitch, and melt it in an 

 iron ladle, and let it boil about -J- of an hour; by this boiling, the pitch, when 

 cold, will become hard and brittle; or you may shorten this operation, by melt- 

 ing equal quantities of pitch and rosin, and then there is no occasion to let it 

 boil so long. The pitch being thus prepared, melt it, and take it off the fire, 

 and let it stand till the pitch becomes pretty cold, or of a thickish consistence; 

 and having warmed the polishing tool a little, to make the pitch stick to it, pour 

 out of the ladle, on the polishing tool, as much of the pitch as you judge will . 

 cover the whole tool; when spread out, to about the thickness of 4 of a" inch; 

 then invert this tool with the pitch on it, and press it on the convex tool, which 



