SPECULUM. 



four or five inches in diameter ; and the reafon is, that if it 

 is much larger, it will enlarge the circle of concavity of the 

 metal worked on rt. The convex tool mull not, however, 

 exceed the fpeculum in diameter more than -rVth of an inch, 

 otherwife it will enlarge the radius of concavity too much. 

 Much water ufed with the bed of hones is detrimental to the 

 figure, and therefore mull be avoided in bringing out the 

 fcratches of the grinding-llone, and in producing a true 

 fpherical face. We have faid nothing hitherto of the fiape 

 of the rough grinder, or convex tool, to be ufed alfo as a 

 pohlher ; this mull be elliptical at its circumference, and 

 the longer diameter mull be to the Ihorter exaftly as lO to 9, 

 the latter being jull equal to the diameter of the fpeculum to 

 be ground and pohlhed by it. This proportion has been found 

 by experience to give the parabolic figure in fpecula of or- 

 dinary dimenfions, by the common mode of ufing the tool 

 in all direftions ; which diicovery by Mr. Edwards gives 

 him a claim to originality of method, as vcell as of fimplicity 

 and certainty in the operations, independently of fuperior 

 flcill in the operator. This elliptical tool is ufed firll after 

 the grinding-llone with fine emery and water, before the 

 bed of hones, and again with the colcothar and water as a 

 polilher, and during the latter operation, it is faid to give 

 alfo the parabolic figure. The preparation of this elliptical 

 convex tool, as a pohlher, is thus effefted. The pitch is 

 made by boiling tar in a ladle or crucible over a very flow 

 fire, till it is very hard when tried cold, and with this an 

 equal quantity of refin is melted, which prevents too much 

 brittlenefs, and makes a good, hard, unelaltic coat for the 

 reception of the colcothar. With regard to the requifite 

 hardnefs of the pitch, Mudge and Edwards feem to differ 

 in opinion ; but as the metal of Edwards is harder than that 

 of the other, and as he ufes a different powder for the polifh, 

 each opinion may be well founded, fo far as relates to his 

 own mode of polilhing. The pitch, or pitch and refin, 

 being melted in a crucible, is poured on the heated elliptical 

 tool, and fpread to the thicknefs of a half-crown, with an 

 iron fpatula ; when it is cooled a little, a Iheet of writing- 

 paper is laid on it, and is prelTed equably with the concave 

 face of the fpeculum itfelf, which does the work of Mudge's 

 concave tool in this refpeft ; and when the paper is removed, 

 it will be fecn, by the fine furrows of its face made thereby, 

 whether or not the impreflion of the fpeculum has, at the 

 firll trial, given its counterpart to the pitch all over ; if not, a 

 fecond imprcfTion mull be taken in like manner, after the 

 pitch has been foftened a little before a fire ; otherwife the 

 face of the fpeculum might have been dipped in water, to 

 prevent adhefion, and then the paper might have been dif- 

 penfed with. The fiiperfiuous pitcli mull now be removed 

 from the edge of the elliptical polillier, and a cone of wood 

 inferted into its central ball, to make it perfeftly round, 

 like the hole of the metal before the coating was poured on. 

 The pitch is next faturated with the colcothar and watev, 

 and then polifliing commences, and is continued, without any 

 particular attention to the Itrokes, more than is necellary for 

 giving the polilh, and when that is finifhed by crofs flrokcs 

 given in all directions, as ufual, the parabolic figure is cn- 

 fured by the variable dimenfions of the polilher'a diameter. 

 Laftly, the final polilh is nearly attaint-d when black mud 

 is generated quickly on the furface of the nutal, which mull 

 be partially, but not entirely, wiped of! at intervals, till the 

 bufinefs is finiflied, when the truth of the figure and good- 

 nefs of the metal may be put to the proof that we have de- 

 fcribed in our former article, as recommended by Mr. Mudge ; 

 or may be tried by Edwards's more convenient tell, which 

 we fhall prefently defcribe. 



Since we made the preceding abridgment of Edwards's 



diredlions, we have feen an extraft from one of his letters 

 to Dr. Maflcelyne, containing an addition to his former direc- 

 tions, which, as it is fhort, we will here fubjoin. " Make 

 the brilliant compofition firll of copper and tin. Melt the 

 proportional quantity of filver and brafs in a fmall crucible 

 by itfelf. When you put the brilliant compofition the 

 fecond time into the crucible, add alfo the lump of brafs 

 and filver melted together before in a feparate crucible ; 

 and when the whole is now fluid, add the proportional 

 quantity of arfenic, and then pour it oft' into the flafks, 

 after the fcoria is taken off, and a httle powdered refin it 

 thrown into it." The reafon afTigned is, that as copper 

 requires more heat to melt it, than either filver or brafs 

 does, if the brafs were put into the high heat of melting 

 copper, its lapis calaminaris would calcine, which wiU 

 not be the cafe when the ingot is made liquid by the 

 lower heat of the fecond melting. The bell fuel that 

 Edwards found for melting the metal was coal-coke, a$ 

 prepared by the malt Hers, which calls no fmoke, and is 

 more lalling than charcoal : he alfo found, that the blueifh 

 hue of crown-glafs ufed for the eye-piece, befl corrects the 

 yellowilh colour of objedls, as fhewn by a fpeculum, and 

 particularly when N° 47 is the compofition chofen. In 

 trying the figure of the great fpeculum, Mr. Edward,; rejefts 

 the diaphragms of Mr. Mudge as unnecefl'ary, and adopti 

 a more fimple method, thus ; when the fpecula and eye piece 

 are in their places, he fixes a circle (uf white paper pro- 

 bably) of half an inch, or an inch in diameter, at the 

 dillance of from fifty to one hundred yards, and gives it 

 a broad black annular margin, by way of contrail ; then, 

 when the telefcope is adjufled nicely to dillinft vifion there- 

 by, the fcrew that moves the fmall mirror mull be turned 

 either way, until a dark haze furrounds the circle, (or field 

 of view,) which will become broader and broader the fur- 

 ther the fcrew is turned ; now, if the haze is more diltinft, 

 and the edge of it better defined when the fcrew turns to 

 the right hand, or brings the fmall fpeculum nearer, from 

 the point of dillinft vifion, than when the motion is in a 

 contrary direftion, the figure of the great fpeculum is 

 fpherical ; but if, on the contrary, the edge of the haze is 

 better defined by the oppolite motion, then the figure is 

 hyperbolical ; and, lallly, if at equal dillances on each fide 

 of^ the true focal point the appearance of the haze is the 

 fame, the figure is known to be properly parabolic, and fit 

 for its office. The fmall fpeculum, in the Gregorian con- 

 ftrudlion, being placed to receive converging rays from the 

 large one, is required to be fplierical, as we have before 

 faid ; but in the Newtonian conllruftion it is made plane, 

 and of an oval fliape, to relledl the rays to the eye, placed 

 at an angle of 90°. Mr. Edwards fays, he received his in- 

 llruftion how to grind and polifli one of thefe from his 

 friend, Mr. (now Dr.) Herlchel, when he lived at Bath. To 

 grind one of ihc rllipiiral hi.'cuh Jlal, a fmall tool of lead 

 with emery is firll ufed, and then two or more confiderably 

 larger ones are ufed ; both the tools and bed of hones 

 fhould not be lef's than fix inches in diameter. Tlie figure 

 of the tools is not conlideied to be completed, till the 

 fpeculum can be firll highly finifhed upon one of them, and 

 afterwards be applied to another, without receiving any 

 change : the lail half dozen llrokes fhould be in the direc- 

 tion of the longer axis of the ellipfis. When this is per- 

 fetted, it mull be poliflied upon the pitch-polilher, of a 

 circular form, the diameter of which is greater tlian the 

 tianfverfc axis of the fpeculum by one-trnlh. As Mr. 

 Edwards fpeaks of Mr. (Dr.) Herfchel as his friend ; and 

 as tlie doftor has not, that we know of, yet publifhed the 

 compofition of liis metal and mode of grinding, poliihinir, and 

 3Q 2 4'guriug 



