S P E 



crofcope makers to preferve fmall objeAs for viewing by 

 ., glafles. The ancients ufed it inllead of glafs in their win- 

 dows. Hill's Hid. of FofTils, p. 72. 



Some take the lapis fpecularis to have been a fpecies of 

 gypfum, and compofed of the acid of vitriol and calca- 

 reous earth. It came into ufe at Rome in the age of 

 Seneca (Ep. 90.), and foon after its introdiiiition, was ap- 

 plied not only to give liglit to apartments, but to protert fruit- 

 trees from the feverity of the weather ; and it is recorded, 

 that the emperor Tiberius was enabled, principally by its 

 means, to have cucumbers at his table during almolt every 

 month in the year. 



Dr. Watfon apprehends it is ftill ufed in fome countries 

 in the place of glafs ; however, it is well known that it 

 was fo ufed in the time of Agricola, for he mentions (De 

 Nat. Fof. lib. v. p. 257.) two churches in Saxony which 

 were lighted by it. Agricola elleemed it to have been a 

 fpecies of plailer-Ilone, and in fpeaking of it he remarks, 

 that though it could bear, without being injured, the heat 

 of fummer and the cold of winter, yet the largell malfes of 

 it were wafted by the rain. However, it differs from 

 plaller-ftone in this property, that it does not, after being 

 calcined and wetted with water, fwell and concrete into 

 a hard llony fubftance. Watfon's Chem. Efl". vol. ii. 

 p. 297, &c. 



SPECULARIA, among the Romans, a kind of win- 

 dow cafements, which were ufed before glafs was intro- 

 duced for this purpofe. They confided of tranfparent 

 ftones, called lapides fpeculares. 



Specularia, the art of preparing and making fpecula, 

 or mirrors : or the laws of mirrors, their phenomena, 

 caufes, &c. called alfo catoptrics. 



SPECULARIS Lapis, in Natural Hiftory. See Spe- 



CULARES. 



SPECULATION. — Certitude of Speculation. See 

 Certitude. 



Speculation Shell, in Natural Hi/lory, a name given 

 by the French writers to a very beautiful fpecies of the 

 voluta, ufually called by us the Guinea-ihell, from its be- 

 ing brought from that part of the world. 



SPECULATIVE Geometry, Mathematics, and Philo- 

 fophy. See the fubflantives. 



Speculative Mufic. By this expreflion is ufually un- 

 derftood fcientific mufic, harmonics, the ratio or propor- 

 tions of found ; in oppolition to praSical mufic, which 

 implies mufic compofed or performed. 



SPECULUM, in Catoptrics, is a metallic refleftor made 

 ufe of in catadioptric telefcopes, inftead of the objedt- 

 glafs ufed in dioptric telcfcopes. 



Newton. — Mr. Newton, afterwards the celebrated fir 

 Ifaac Newton, had difcovcred that light conjijls of rays ,lif- 

 ferently refrangible, and that confequently no figure, which 

 could be given to a fingle piece of glafs, would make ail 

 the tranfmitted rays concentrate in the focus of that glals 

 ufed as an objcft-glafs : he n-linquifhed his glafs-workii, and 

 immediately turned his mmd to the confidcration of what 

 could be done in the condruftion of a tcleicope, where 

 the rays of light were made to concentrate by reflcdtion ; 

 having prcvioully difcovercd and proved, that the angle ot 

 refleftion of all forts of rays was equal to the angle of in- 

 cidence, without tile lead perceptible difpeition. Uniler 

 an imprefiion that an inllrument, founded on this bafis, 

 migiit be made more perfctl than could be made with 

 glalles alone, when ufed in the ufual way, he tried luch 

 metallic fubttances as promifed to prjiUice a hard and 

 bright texture, and to admit of a good polifh, and in 



SPE 



February, 1672, laid before the Royal Society an account 

 of a telefcope of an entire new condruftion, that gave a 

 didnift image, and magnified or increafed the vifual angle 

 thirty-eight times. It is fomewhat remarkable, though not 

 fo much to be wondered at, when we confider the genius 

 of the inventor, that this fird attempt detefted not only 

 the bed materials, but the bed mode of fufing and mixing 

 them, if not in the bed proportions ; and left fucceeding 

 indrument-makers little more to do than to vary the pro- 

 portions, and to exemphfy the theory which he pomted out 

 by a fucceflion of nice and flcilful manipulations. Copper, 

 tin, and a portion of arfenic, with fometimes a little filver, 

 were then, as they are at this time, the eilential ingredients 

 of the compound metal that condituted the bed fpeculum. 

 The ingenious Huygens was no fooner informed of the 

 occafion and fuccefs of this projeft, than he calculated and 

 candidly acknowledged the advantage that a good con- 

 cave fpeculum mud have over convex glafles in concen- 

 trating all the different rays of heterogeneous light. With 

 this fird indrument the fatellites of Jupiter, as well as the 

 lune of Venus's difc, were clearly perceived. The fecond 

 telefcope, however, was more perfedf, and a page of the 

 Philofophical Tranfaftions could be didindly read with it 

 at the didance of ico feet. In this condruftion, which 

 has obtained the name of the Newtonian conftruAion, there 

 were two fpecula : one fpeculum, which was ground to a 

 given radius of curvature, was placed at the end of the 

 tube, which was remote from the objeft to be viewed, and 

 the other, which was fmall, was plane, and was fixed in an 

 angle of 45° in the axis of vifion, fo as to refleft the con- 

 verging rays to a focus, at the focus of a fmall eye-glafs 

 placed at the fide of the tube, not far from its anterior end 

 or aperture where the eye was fituated to view the objeft, 

 that aftually lay to the left hand. Cad'egrain in the fame 

 year communicated to the Royal Society the figure of a 

 reflefting telefcope, where a perforation in the large fpecu- 

 lum admitted the eye-piece to be placed as in the dioptric 

 telefcope, which he thought had feveral advantages, but he 

 did not detail any procelTes he had ufed towards attaining 

 fuch a condruftion ; in reply to which fir Ifaac informed 

 him, that Mr. Gregory had defcribed a fimilar condruftion 

 in his " Optica Promota," in the year 1663, from which 

 reply it fhould feem, that refieSion had been confidered by 

 Gregory before fir Ifaac condrufted his telefcope, fo that 

 the Gregorian condruftion mud be confidered as preceding 

 the Newtonian, fo far as relates to the arrangement of its 

 parts ; but we are not to conclude from thence, that the 

 bell compofition for the fpeculum was not fir Ifaac New- 

 ton's. During the correfpondcnce that Huygens had with 

 our author, on the fubjeft of a metallic fpeculum, he took 

 occafion to obferve, that no doubt the fuperiority of a 

 parabolic curve for the face of the reflefting metal over a 

 ipherical one mud be known to him, and fir Ifaac was of 

 opinion that fome mechanical device might be found out, 

 to produce the dclired curvature, belter than any of the 

 conic ftUions ; that is, as we fuppufe, better than any tool 

 formed agreeably to one of the conic feftions : and with 

 this view Mr. Stephen Gray, we find, prefented a projedt of 

 forming a grinding tool of a concatenary fhape to be fub- 

 ditutcd for the parabola, which, however, did not anfwer 

 in praftice. The projeft was this ; a piece of clay was 

 formed thin, when foft, into an cxaft circular cake, and 

 laid on a fufpended horizontal rim or ring of a fmaller dia- 

 meter, fo as to allow the cake to aflnme the fhape of the 

 required curve, which it appeared to do ; but on baking 

 this convex cake, it was found to have altered its figure, 



and 



