GiO 
OPT 
When the metal is caft, and prepared by the common 
grindftone for receiving its proper figure, the gages and 
grinding-tools are to be formed in the fame manner as for 
convex- lenfes, with this difference only, that the radius 
of the gages mu ft always be double the focal length of 
the fpeculum. In addition to the convex and concave 
brafs tools, which fhould be only a little broader than the 
metal itfelf, a convex elliptical tool of lead and tin fhould 
aifo be formed with the fame radius, fo that its tranfverfe 
may be to its conjugate diameter as io to 9, the latter 
bring exa< 5 Ily equal to the diameter of the metal. On this 
tool the fpeculum is to be ground with flour-emery, in 
the fame manner as lenfes, with circular and crofs ftrokes 
alternately, till its furface is freed from every imperfec¬ 
tion, and ground to a fpherical figure. It is then to be 
wrought with great circumfpettion, on the convex brafs 
tool, with emery of different degrees of finenefs, the con¬ 
cave tool being fomefimes ground upon the convex one, 
to keep them all of the fame radius ; and, when every 
fcratch and appearance of roughnefs is removed from its 
furface, it will be fit for receiving the final poliih. Before 
the fpeculum is brought to the polifher, it has been the 
practice to fmootli it on a bed of hones, or a convex tool 
made of common blue bones. This additional tool, in¬ 
deed, is abfolutely neceflary, when filver and brafs enter 
into the compofition of the metal, in order to remove that 
roughntfs which will aiu'ays remain after the fineft emery 
has been ufed ; but, when tliefe metals are not ingredients 
in the fpeculum, there is no occafion for the bed of hones. 
Mr. Edwards does not ufe any brafs tools in his procefs, 
but transfers the metal from the elliptical leaden tool to 
the bed of hones. By this means the operation is fimpli- 
fied ; but we doubt much if it is in the leaft degree im¬ 
proved. As a bed of hones is more apt to change its form 
than a tool of brafs, it is certainly of great confequence 
that the fpeculum fhould have as true a figure as poflible 
before it is brought to the hones; and we are perfuaded, 
from experience, that this figure may be better commu¬ 
nicated on a brafs tool, which can always be kept at the 
fame curvature by its correfponding tool, than on an el¬ 
liptical block of lead. We are certain however, that, 
when the fpeculum is required to be of a determinate fo¬ 
cal length, this length will be obtained more precifely 
with the brafs tools than without them. But Mr. Edwards 
has obferved, that thefe tools are not only unneceffary, 
but “ really detrimental.” That Mr. Edwards found 
them unneceffary, we cannot doubt, from the excellence 
, of the fpecula which he formed without their afliftance ; 
but it feems inconceivable how the brafs tools can be in 
the leaft degree detrimental. If the mirror is ground 
upon twenty different tools before it is brought to the bed 
of hones, it will receive from the laft of thefe tools a 
certain figure, which it would have received even if it had 
not been ground on any of the reft; and it cannot be 
queftioned, that a metal wrought upon a pair of brafs 
tools is equally, if not more, fit for the bed of hones, 
than if it had been ground merely on a tool of lead. 
When the metal is ready for polilhing, the elliptical 
leaden tool is to be covered with black pitch, about one- 
twentieth of an inch thick, and the polilher formed in the 
fame way as in the cafe of lenfes, either with the concave 
brafs tool, or with the metal itfelf, The colcothar of vi¬ 
triol fhould then he triturated between two furfaces of 
glafs, and a confiderable quantity of it applied at firft to 
the furface of the polifher. The fpeculum is then to be 
wrought in the ufual way upon the polifhing-tool, till 
it has received a brilliant luftre, taking care to ufe no 
more of the colcothar if it can be avoided, and only a 
fmall quantity of it if it fhould be found neceflary. 
When the metal moves ftiffiy on the polifher, and the col¬ 
cothar affumes a dark muddy hue, the polifh advances 
with great rapidity. The tool will then grow warm, and 
would probably ftick to the fpeculum if its motion were 
difcontinued for a moment. At this ftage of the procefs, 
therefore, we muft proceed with great caution, breathing 
continually on the polifher, till the friction is fo great as 
I c s. 
to retard the motion of the fpeculum. When this hap¬ 
pens, the metal is to be flipped off the tool at one fide, 
cleaned with foft leather, and placed in a tube, for the mr- 
pofe of trying its performance ; and, if the polifhing has 
been concluded with care, it will be found to have a true 
parabolic figure. Appendix to Fergvfott's Lectures, vol. ii. 
Whoever defires to have a perfect and fine in Tument, 
muft have at leaft two fets of fpecula, made: this alone 
will give the optician a fair chance of doing his belt; for 
fuch is the extreme uncertainty of obtaining a perfect 
figure, that, if their employers are not liberal enough to 
pay for the extra labour, they ought not to be furprifed 
if the makers are willing to ftop when the figure is tolera¬ 
bly good, rather than run the rifk of deftroying a week’s 
work, by trying to make it a fine one. Get one metal as 
good as you can, then fet to work at another, and, when 
you have made one more perfeft, try to mend the firft : 
thus, by alternately working one after the other, you will 
at laft obtain the ne plus ultra of perfeCfion. 
At a recent fitting of the firft clafs of the French Infti- 
tute, M. Rochon prefented an objeCt-glafs of Dollond’s, 
four inches in diameter, which had been broken in pieces 
and repaired by him. It was a triple achromatic one, of 
the lenfes of crown-glafs, was broken into a great num¬ 
ber of pieces, and fome of them loft, the other remained 
whole ; the lens of flint : glafs was compofed of twenty- 
two triangular fegments, of different lizes, terminating 
near the centre. Thefe fegments were carefully adjufted 
on the whole crown-lens, and a new lens of crown-glafs 
made to replace the one of which the pieces were loft; 
the three glaffes were cemented together with oil of tur¬ 
pentine, as well to diminifti the inevitable irregularities 
of the adjuftment of the broken lens, as to maintain 
them in their pofition. Experiments have been made 
with the glafs thus repaired ; and, lingular to relate, in 
even the fineft obfervations, as a teft of the fuccefs, it did 
not reflect a double image ; from whence it is apparent, 
that, in cafes where the lofs of an objeCt-glafs cannot be 
repaired by a new one, it may be worth the while to effay 
what may be done with the,old broken one, the fuccefs- 
ful experiment of M. Rochon being in proof that fuch a 
ftep is practicable. 
It is of the utmoft confequence to the perfection of re¬ 
flecting telefcopes, that the mirrors be truly parallel to 
each other, and alfo that the centres of them, together 
with the centres of the eye-glaffes, be all in one direfl 
line; viz. in the axis of the tube. Indeed, uniefs thefe 
particulars are attended to, the inftrument will prove de¬ 
fective and faulty, even though the mirrors have the raoft 
exquifite figure poflible given to them. That truly ex¬ 
cellent artilt, the late ingenious Mr. James Short, always 
took the greateft care to adjuft and centre the metals of 
his telefcopes. If the mirrors are truly centred and ad¬ 
jufted to their beft pofition, a fixed ftar, when the tele- 
fcope is put out of focus, Ihould always appear, in reflect¬ 
ing telefcopes, as a truly round circle of fire with a black 
fpot exaCtly in its centre ; and, when the telefcope is ad¬ 
jufted to diftinCt viiion, the ftar fiiould appear, if the te¬ 
lefcope is excellent, and the ftate of the air favourable, 
exaCtly round, and totally free from all irradiations, or 
falle rays and glare. Indeed, no objeCl is fo proper to 
determine the excellence of telefcopes as the fixed ftars, 
as the leaft irregularity in the figure of the metals in re¬ 
flecting telefcopes, or of the objeCt-glafs in achromatics, 
is rendered by them exceedingly confpicuous by a falfe 
glare, and by their not appearing perfectly round. 
There is an extraordinary and curious faCt, with which 
few people are acquainted, but is of the firft importance 
every one Ihould be aware of when choofing a telefcope, 
or comparing inftruments to afcertain their peculiar 
powers ; that, when trying aftronomical glaffes, we Ihould 
not be fatisfied with lefs than three evenings -obfervation. 
Such is the capricious variation of the atmofphere of this 
country, that fome evenings which appear extremely fine, 
when the ftars look molt brilliant and dazzling to the 
naked eye* are quite unfit for obfervation, and our beft 
telefcopes 
