VOL. LXVir.] PHILOSOPHICAL TRANSACTIONS. 173 



polisher, and let the whole rest in this state 2 or 3 hours : this will produce an 

 intimate contact between the two, and by parting, with any degree of warmth 

 they may have acquired by the vicinity of the operator, they will become per- 

 fectly cold together. By this time you may push out the cork from the polisher, 

 to discharge the water, and give tiie metal the parabolic figure in the following 

 manner. Move the metal gently and slowly at first, a very little round the 

 centre of the polisher (indeed after this rest it will move stiffly) then increasing 

 by degrees the diameter of these strokes, and turning the metal fretjueiitly round 

 its axis, give it a larger circular motion, and this without any pressure but its own 

 weight, and holding it loosely between the fingers : this manner of working may 

 safely be continued about 2 minutes, moving yourself as usual round the block, 

 and carrying the round strokes in their increased and largest state, not more than 

 will move the edge of the metal half an inch or a over the tool. The speculum 

 must not all this while be taken oft' from the polisher ; and consequently no 

 fresh putty can be added. It will not be safe to continue this motion longer 

 than the time abovementioned ; for if the parabolic tendency be carried the least 

 too far, it will be impossible to recover a true figure of that kind but by going 

 through the whole process for the spherical one in the manner before described, 

 by the cross strokes on the polisher, which takes a great deal of time. How- 

 ever, when there is occasion, it may be done ; and I have myself several times 

 recovered the circular figure, when I had inadvertently gone too far with the 

 parabolic ; and ultimately finished the metal on the polisher without the use of 

 the hones. 



To try the true figure of the metal. — It will now be proper to try the figure of 

 the speculum, and that is always best done by placing it in the telescope it is 

 intended for. In order to this, I use the instrument as a kind of microscope, 

 placing the object however at such a distance that the rays may be nearly parallel. 

 At about 20 yards a watch-paper, or some such object, on which there are some 

 very fine hair strokes of a graver, is fixed up. The lead must be then taken 

 off from the back of the speculum ; which is best done by placing the edge of a 

 knife at the junction of the lead and metal, when, by striking the back of it 

 with a slight blow, the pitch immediately separates, and the handle drops off"; 

 the remaining pitch may be scraped off' with a knife, taking care that none of 

 the dust stick to the polished face of the metal. 



Having placed the speculum in the cell of the tube, and directed the instru- 

 ment to the object, make an annular kind of diaphragm with card-paper, so as 

 to cover a circular portion of the middle part of the metal between the hole and 

 the circumference, equal in breadth to about an 8th part of the diameter of the 

 speculum : this paper ring should be fixed in the mouth of the telescope, and 

 remain so during the whole experiment, for the part of the metal covered by it 



