252 Mr Sang's Notice of a 



out a series of grooves parallel to the axis : the ridges between 

 these were removed by using intermediate stops on the lathe- 

 spindle ; and the whole was then smoothed out by a cylinder 

 of lead with fine emery : it was then ground and polished in the 

 usual way for hollow cylinders. 



In order to cut the outside to the proper form, the lens thus 

 bored out, was chucked on a turned block of lead, which had 

 been cast on an iron mandril ; the surface of the lead being pre- 

 viously covered by a fine thread which was nowhere doubled, 

 l^y this means the axis of the interior surface was made coinci- 

 dent with that of the lathe. 



The evolvent of the required curve was then computed, and 

 the edges of two plates of cast-iron carefully formed to it. 

 These are seen attached to the cheeks of the accompanying 

 frame, and are represented in the drawing. Two pieces of 

 watch-spring were then made to bend over them, so that on un- 

 coiling the springs, one point in each would trace over the 

 curve wanted : these springs were then attached to the ends of 

 an axis by adjusting pieces, and that axis had its parallelism 

 preserved by means of a jointed frame. 



This apparatus so prepared, was fixed on the bed of the 

 turning-lathe, and the moveable axis rendered perpendicular to 

 the axis of the lathe. On this moveable axis there were placed 

 the cutters and polishers ; it received motion by means of a 

 small pully fixed on it near one end. 



To make the first approximation to the shape of the lens, an 

 iron cylinder was fitted on the moveable axis, and its surface 

 was primed with diamond. The lathe-spindle being still, this 

 iron cylinder was brought over the glass, so as to cut part of a 

 cylindroid surface, whose base was nearly the required curve ; 

 the lathe-spindle then being moved a division round, another 

 surface was cut, and this was continued all round ; as soon as 

 a sufficient approximation was made, both' spindles were set to 

 revolve at once, and the cutting continued till a uniform sur- 

 face was produced. The iron tool with diamond was then re- 

 placed by a tin one with fine emery, and the separate motions 

 were varied and reversed so as to produce every variety in the 

 direction in which the surfaces met each other. Lastly, the tin 

 tool was removed, and its place supplied by one of willow, the 



