S P E 



figuring it ; we may conclude that hepraaifes Mr. Edwards's 

 procefs, and particularly as Mr. Edwards gives us the fol- 

 lowing information in the poftfcript of his pamphlet. " Mr. 

 Herfchel chiefly makes ufe of a Newtonian refleaor, the 

 focal diitance of whofe great mirror is feven feet, us aper- 

 ture 6.25 inches, and powers227 and 460 times, though fome- 

 times he ufes a power of 6450 for the fixt ftars. Note, it 

 the metals of a Newtonian telefcope are worked as exqui- 

 fuely as thofe in Mr. Herfchel's feven-feet refledors, the 

 higheil power that fuch a telefcope fhould bear, with 

 perfea diftinanefs, will be given, by multiplying the dia- 

 meter of the great fpeculum by 74 ; and the focal diftance 

 of the Jingle eye-glafs may be found, by dividing the focal 

 diftance of the great mirror by tiie magnifying power : 

 thus, 6.25 X 74 = 462, the magnifying power ; and 



7 X 12 

 462 



0.182 of an inch, will be the focal length of the 



fingle eye-glafs required." When we arrive at our article 

 Telescope, we truft that we (hdl be able to lay before our 

 readers more minute information refpeaing the fpecula, as 

 well as other parts of Dr. Herfchel's wonderful inltruments, 

 than we have room for in our prefent article. 



Suppofing now Mr. Edwards's fpecula to be finiftied, it 

 remains that we fay a few words refpeaing his mode of 

 fixing them in his tube, to give a diftinft image of the ob- 

 jea to be viewed, without which they would be ufelefs. 

 When the large fpeculum is made to reft againft the inner 

 end of the tube for which it is adapted, the arm which car- 

 ries the fmall fpeculum muft be firft adjufted : in order to 

 do this properly, extend firft two fine threads, or wires, 

 acrofs the aperture of the tube at right angles, fo as to 

 interfea each other exaaiy in the axis of the tube ; before 

 the arm is finally fattened to the fiider, place it in the tube, 

 and through the eye-piece, without glaffes, let the eye be 

 direaed to the fcrew-hole in the end of the arm, and if the 

 point of interfeaion of the crofs-wires be at its centre, the 

 hole is in the axis, but not otherwife. When this exaanefs 

 is obtained by mechanical means, the arm may be there 

 made faft to the Aider. In the next place, the fmall fpe- 

 culum muft be placed parallel to the large one, thus ; cut 

 a piece of card into a circle about one-half or two-thirds of 

 the diameter of the mouth of the tube, and fix it concen- 

 trically at the mouth with a piece of ftick, that fhall bifea 

 the tube, to the middle of which the circular cord may be 

 tied, fo as to exclude all the light, except an annular por- 

 tion at the circumference of the tube ; this being done, 

 turn the telefcope to the iky, and the annular light that 

 falls on the great fpeculum will be refleaed on the little 

 fpeculum, on which it will form an annular light alfo of 

 fmaller dimenfions. Now, if the breadth of this fmall ring 

 of light is obferved to be equally broad at every fide of the 

 fmall fpeculum, the pofition thereof is parallel to that of 

 the large one, but if not, it muft be adjufted till this is the 

 cafe. But it may fo happen, that this adjuftment may 

 put the centre of the fmall fpeculum out of the axis of the 

 tube, which may be deteaed thus ; examine again the 

 annular light, and take notice whether or not the ring is 

 concentric with the circumference of the metal ; if not, it 

 muft be made fo by the fcrews of adjuftment, or otherwife. 

 In order to be certain that thefe adjuftuients of the fmall 

 fpeculum are accurate, turn the telefcope aijain to the fmall 

 circle of paper with a black margin, with the tube quite 

 open, and having made its image concentric with the fmall 

 fpeculum at the point of diftina vifion, alter the focus by 

 moving the fmall fpeculum gradually, till the black margin 

 becomes a black fpot, or point ; and if this fpot lie on the 



S P E 



centre of the fmall fpeculum apparently, the two adjuft' 

 ments, for the axis of vifion, and for parallelifm, are both 

 correa. Again, if a ftar of the firft magnitude appear 

 perfeaiy round and well defined ; and alfo if the fmall 

 itars, in the double ftars, are well fsen, thefe are proofs 

 of the excellence not only of the figure and polifh of the 

 metal, but likewife of all the adjuftments. 



We have only to add further to what we have faid of Ed- 

 wards's metal and procefs, that on conferring lately with fome 

 of the moft eminen; opticians of the prefent day, their opinion 

 is, that refl?aing telefcopes are brought as near to perfedion 

 as art and experience can effea, by Watfon, TuUey, and 

 others, but that it is not neceifary to adiiere rigidly to Mr. 

 Edwards's, any more than to Mr. Mudge's direaions ; the 

 Edgedon ftones, for inftance, do not anfwer the defcription 

 given of them for cutting the hardeft metals, which are 

 always the beil when the colour is proper ; (the denfity of 

 which, we underftand, may be determined by their fpecific 

 gravity, without the trouble of previous polilhing) ; neither 

 is a bed of hones indifpenfably necefTary ; as the fineft 

 emery, that wafhes off^ in grinding, may be again ufed to 

 give a fmooth furface before poliihing, as the TuUeys, father 

 and fon, have fometimes done, perhaps more out of curiofity 

 than choice, and have thus begun and finifhed the procefs 

 with only one tool, unlefs the fpeculum itfelf be called a tool 

 for equalizing the pitch. Lord Stanhope, and his operator, 

 Mr, Varley, who are engaged in making experiments in 

 polilhing fpecula, have uled Welih flate with fuccefs for 

 compofing a bed of polilhing materials ; and no doubt there 

 are various ftones not tried that may hereafter prove fuperior 

 to thofe hitherto ufed for this purpofe. The brafs and 

 filver recommended by Edwards are, moreover, now gene- 

 rally omitted by the beil opticians, as tending to diminilh 

 the hardnefs of the metal more than is requifite ; but the ar- 

 fenic is ftill retained. And, laftly, the property of the eUip- 

 tic tool, afcribed to it by Edwards, is difallowed by feveral 

 of the moft fl<ilful opticians ; fo that, after all, each work- 

 man fucceeds bell when he praaifes the methods of grinding, 

 poliftiing, and giving the requifite figure, in which his own 

 (kill and experience have rendered him moft expert, and 

 confequently molt confident. 



Speculum, Horizontal. See Horizontal. 



Speculum. This name is given to feveral inftruments 

 employed in Surgery, for gaining a view of tumours, or 

 ulcers, fituated within cavities. Thus, there are fpecula ani, 

 fpecula auris, &c. The fpeculum ocuh is an inftrument for 

 opening the eye-lids, and keeping them apart, fo that the 

 furgeon may have a good view of the eye. 



Speculum Mtiftci, the title of a mufical treatife, written 

 in Latin by John de Mnris, (fee MuRis,) and only to be 

 found among the MSS. in the late king of France's 

 library, at Paris. It is a treatife fo ample, and fo fcarce, 

 that, having procured large extraas from it, and a com- 

 plete table of its contents, we (hall be the more minute in 

 our account of it, as it feems to have been the ground-work 

 of all the mufical treatifes that were produced by other 

 writers, till the time of Franchinus Gaforius, in the latter 

 end of the 15th century. 



This work, which is written on vellum, in folio, contains 

 fix hundred pages. The firft fentence of the original is, 

 " Libro tertio de Philofophica Confolatione Boetius volens 

 reddere Caufam," &c. It is divided into feven books : the 

 firft of which treats of the invention of mufic, and of its 

 divifions, and contains 76 chapters ; the fecond, of mufical 

 intervals, 123 ; the third, of harmonics, or mufical propor- 

 tion, 56; the fourth, of concords and difcords, 51 ; fifth, 

 of the ancient tetrachords, divifion of the monochord, and 



doarines 



