A SILVERED GLASS TELESCOPE. 25 



the foot-power. At one end of the primary shaft is firmly fixed the cogwheel k, 

 which drives the crank-shaft I. Attached to the horizontal part of I, is the crank- 

 pin m. The two bolts n n' move in a slot, so that the crank-pin may be set at any 

 distance from to 2 inches, out of line with I. Above, the crank-pin carries one 

 end of the bar o, the other end passing through an elliptical hole in the oak-block 

 p. Down the middle of the bar runs a long slot, through which the screw-pin q 

 passes, and Avhich permits q to be brought over any zone from the centre to the 

 edge of the mirror a. It is retained by the bolts r r', which are tapped into s. 

 The local polisher is seen at t. The curve which the centre of the local polisher 

 describes upon the face of the mirror, varies with the adjustments. Fig. 26 is a 

 reduction from one traced by the machine, the overlapping 

 being seen on the left side. The mirror is not tightly con- .^-^^^^ 



fined by the cleets cc', for that would certainly injure the 

 figure, but performs a slow motion of rotation, so that in 

 no two successive strokes are the same parts of the edge 

 pressed against them. 



The local polishers are made of lead, alloyed with a smaU. 

 proportion of antimony, and are 8, 6, and 4 inches in di- 

 ameter, respectively. The largest and smallest are most 

 used, the former on account of its size polishing most „ , ., , ^ 



'- ~ Hypocycloidal Curve. 



quickly, but the latter giving the truest surface. The rosin 



^that covers them is just indentable by the thumb nail, and is arranged in a novel 

 manner. The leaden basis, as seen at t, Fig. 25, is perforated in many places with 

 holes, which permit evaporation, serve for the introduction of water where needed, 

 and allow the rosin to spread freely. Grooves are made from one aperture to 

 another, and the rosin thus divided into irregular portions. The effects of the pro- 

 duction of heat are in this way avoided. 



The mirror may be ground and fined on this machine, in the same manner as on 

 that described at page 21, or it may be ground with a small tool 8 inches in 

 diameter, as recently suggested by M. Foucault, the results in the latter case being 

 just as good a surface of revolution as in the former. It is best polished with the 

 8 inch, and a moderate pressure may be given by the screw q, if the pitch is not 

 too soft. This, however, tends to leave an excavated place at the centre of the 

 mirror, the size depending on the stroke of the crank m, which should be about 2 

 inches. The pin q ought to be half way from the centre to the edge of the mirror, 

 but must be occasionally moved right or left an inch along the slot. When the 

 surface is approaching a perfect polish, the warmed 4 inch polisher must be put in 

 the place of the 8 inch. The pin q must be set exactly half-way between the centre 

 and edge of the mirror, and the crank must have a stroke of two inches radius. 

 The polisher then just goes up to the centre of the glass surface with one edge, 

 and to the periphery with the other, while the outer excursion of the inner edge 

 and inner excursion of the outer edge meet, and neutralize one another at a mid- 

 way point. Wherever the edge of a polisher changes direction many times in 

 succession, on a surface, a zone is sure to form, unless avoided in this manner. , All 

 the foregoing description is for a 15g inch mirror. 



4 June. 18Q4, 



