w 



ill 



638 



REroRT— 1884. 



iji; 



.;-« 



■witliso iiiiicliiliiliciilty, that it Ikh occurred to mc; tliat, iiltlioiiijli my own I'xiicrii'iK'i' 

 is fur iiifiirior to tliiit of my lute latiier, a lew observiitioiis may lie of iiiU-nst. 



The ])r()('('ss lor prcpiiriiif,'' iiiul comiilotliiL,' ii jtaraLolio sjirciiliiin, aflcr lluit tin- 

 castiiiL,'-, roii^'li [^Tiniliiii;', Sec, have Ihcii coiniilcled, cdiisists of two operations : lii-^it, 

 the 'frriiiiliii;j: " with eiiu'ry of liner ijuality and Hinallcr (juantily as tlii> opcralioii 

 proceeds, hy wliicli so line and true a splierical .-nrl'aco may lief^iven as may ([uicidv 

 receive a polish under tlio action of tlio polislior; and, pec(m(ily, tlie 'polisluu^'' jToper. 

 wliicli not only iinjiarls the polish ])nt chaiii^es, hy the action of the elustie surfaou 

 of ])iteh covered with ii ])aste of ron;j:e and water, the spherical into the ])aral)(i]oi(l 

 surface The prin<'ipal ciiuses of uncertainty in the pr(jcess are the varialih; liyirm- 

 metric condition and temperature of tlieair. If the air Le too moist, tliat j^radiial 

 drying-up of the moisture of the polisiiiuf,' material which is essential to success 

 will not lake place ; if, on tlie other hand, it he too dry, it will be ditiicult ti> 

 preserve that uniformity of moisture wliich is equally desirahle. Afrain, if tiie 

 temperature ))e too liin'h for the fjuality of ])itch eni])ioyed, the surfiice will he 

 untrue in detail from undue wearinjr down of tlio softer parts, and the ])itch willhr 

 pressed out too thin hel'on! the process is complete ; while, if the pitch ]w too hard, 

 the rnuu'e will not become siilliciently embedded in it, and the polish will In- 

 imperfect, and also the pitcli will not be ubhi to adapt itself t(j thi> speculum, ami 

 an imperfection of tjirure will result, lu short, we have three variables — ieni|ii- 

 rature, moisture, an<l quality of jiitch, and these must be so relatively adjusted sis 

 to give a satisfactory result. Two kinds of stroke were employed by my father, 

 that known .he straight stroke, produced by the action of two eccentrics, one 

 making many (suy lo) revolutions to fine of the other, and that known as the 

 circular stroke, euiployed also by .Mr. Lassell, where both eccentrics rotated in 

 equal times, their radii being ])arallel to one another. 



The xfraif/Zif. stroke involved a considerable ov(n'bang of the pidisher during twn 

 or three consecutive strokes, and a consequent difficulty in ])reventing too great local 

 compression of the pitch, while there was some dilUculty in preventing an unequal 

 eva]>oration of llu; nuii.rture when the r.ir was a little too dry. 



The circular stroke also involved a tender.cy to une((ual drying, and a tendenc_\ 

 to annular elevations, and depressions of the surface of the .'speculum could oidy be 

 removed by a change at intervals of the distance from the centre of the circle de- 

 scribed by the centre of the polisher from the centre of the speculum. This was 

 etlected in the case of the .'i-foot speculum by a sonu'what complicated contrivance, 

 and in the case of the G-foot speculum simply by hand. 



The late I\Ir. Jjassell, in the 'Phil. Trans, for IS"."),' describes a miichinr 

 in which i\w relative angular motions of the eccentrics dill'er a little only, but as 

 at the tinu! he bad nearly given up observing owing to age and inlirmlty, the 

 machin(> probably was not tested as it deserved. 



J have, during the last two years, reconstructed the two polishing machines at 

 Parsonstown, and have adapted tlieni for giving to the polisher the stroke of Mr. 

 La.ssell's machine.' I have polished two o-foot specula and one 0-foot specuhnn. 

 Ill each case the result has been much above the average. There has been no ap- 

 preciablt> imiHpial drying, and no tendency to annular depressions. 



It should be observed that in all the machines the speculum has a slow move- 

 ment of rotation on its a.xis, and also the polisher turns round, at a dilferent rate. 

 This was ciiected by Lassell by means of a ratchet. "We have found no necessity 

 for a special contrivance, as the poli,«her binng grasped by a loose hoop, with 

 perhaps j 1o 1 incli of play, I'otates by rolling inside the hoop. 



With i-egard to the polishing of Hat mirrors, as the required figure can be 

 imparted in the grinding, it appears to be much the simplest to take pains to 

 obtain as Hat a surface as possible before polishing, and to impart as fine - a surface 

 also as we can in the grinding. Care in this is well repaid by the shortness of time 



' On reference to Mr. La.sseH's paper, it appears that the relative angular velocities 

 o£ his eccentrics arc as 1 : 01)22. In my maobino the relative velocities an; a.s 

 1 ■; 0806, but by means of cbango wheels .any otlier relative vi'locities can be given. 



'^ A bed of bones, made by cementing pieces of Welsh bones on to a metal plate, 

 is well adapted for giving a tine surface before polishing. 



