A SILVERED GLASS TELESCOPE. 25 



the foot-power. At one ciid of tl,o primary sliaft is firmly fixed the cogwheel /-•, 

 which drives the crank-sliaft /. Attached to tlie liorizontal part of /, is the crank- 

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

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

 end of the luir o, the otlier end passing tlirongli an elliptical hole in the oak-block 

 J). Down the middle of the bar rnns a long slot, throngh which the screw-pin q 

 passes, and wliic'h permits y to be brought over any /oih^ from the centre to the 

 edge of the mirror a. It is retained by the bolts ;•/•', wliicli arc tap])ed into s. 

 The local polisher is seen at /. 'I'iie curve wliicli tlu^ centre of the local polisher 

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

 reduction from one tracu'd by the machine, the overla])ping 

 being seen on the left side. The mirror is not tightly con- ^^1^^^-^ 



fined by the cleets c r/, for that woidd certainly injure the 

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

 no two successive strokes are the same parts of tlie cd'j^c 

 pressed against them. 



The local polishers are made of lead, alloyed with ;i small 

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

 ameter, respectively. The largest and smallest ;ire most 

 used, the former on account of its size polishin"- most „ , -j , r. 



I " Hypocycloidal Cutve. 



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 /, 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 eftects 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, tlie results in the latter case being 

 just as good a surface of revolution as in tln^ former. It is bi^st polished with the 

 8 inch, and a nuderate pressure may be gi\-en by the screw r/, 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 i/t, 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 l(>ft an inch along the slot. AMien 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 nnist be set exactly half-way between th(^ 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, Avliile 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 polish(>r changes direction many times in 

 succession, on a surfa<;e, a zcmc is sure to form, unless avoided in this manner. AH 

 the foregoing description is for a 15^ inch mirror. 



4 June, 1864. 



