OPT 
From thefe tables our Ikilful optician takes his curves, 
by infpe&ion, fuitable for glafs of any given fpecific gra¬ 
vity, fuch as will fuit his tools for telefcopes of different 
lengths; and, having as it were thecommand of the whole 
range of varying ratios, he can immediately fix on fuitable 
curves for any glafs, and for any compound focal length, 
or even afligti a fellow that fiiall match any practicable lens, 
convex or concave, that has been previoufly polifhed. 
Such is the facility which this ingenious and perfevering 
optician has attained in the higheft branch of his art, 
whilfl, at the fame time, his (kill in grinding, polifhing, 
and centreing, his glades, is not exceeded by any other 
artift. The principal deviation from Martin’s rules that 
Tulley found it neceffary to adopt in his pratlice, is the 
application of a correcting number to the calculated or tabu¬ 
lated aberration arifing from the figure of the flint-glafs, on 
account of its difference of refractive power, as compared 
with that of the crown-glafs ; in order to gain which cor- 
reCling number in all different cafes, he firfl reduces the 
f ;eometrical foci of the two feparate lenfes into the re- 
raCled foci by his divifor= Martin’s 2 a, and extraCls the 
fquare-root of the cubes of thofe refraCled foci refpeClive- 
ly ; then dividing the root of the flint-glafs by the root of 
the crown-glafs, he gains the correcting divifor, by which 
the calculated aberration of the flint-glafs is divided, to 
produce the corrected aberration for the concave lens; 
which lens rauft now have its radii determined agreeably 
to this corrected aberration from the general theorem, or 
may be taken from the tables to be fubflituted for the radii 
that would have been requifite if the proportional aber¬ 
ration had remained uncorrefled. And laftly, that the 
foci of the feparate lenfes may be fo proportioned to each 
other, and to the compound focus of both the lenfes, 
which is ufually given when a telefcope is to be made, 
the ratio between the focus of the crown-glafs and of the 
compound glafs, having been calculated by an appropriate 
theorem, as will be explained, is tabulated to fuit differ¬ 
ent forts of glafs, agreeably to their fpecific gravities; fo 
that Martin’ $ conllant ratio of 5 to 3 is varied accor¬ 
ding to the variation of the fpecific gravity, which is 
afl'umed as bearing a due proportion to the difperfive 
power. 
Befides the preceding improvers of the telefcope, feve- 
rai perfons, chiefly amateurs, have taken out patents, ei¬ 
ther for alterations in the appendages of this inArument, 
or for peculiar modes of tiling them for particular purpofes, 
with a fhort notice of which we fliall conclude this feflion 
of our article. 
On the 4th of April, 1791, Mr. Robert Blair, a furgeon 
in the flavy, took out a patent for fecuring to himfelf the 
advantages to be derived from ufing a fluid medium, in 
conjunction with glafs, to correct the prifmatic aberration 
in an objeif-glafs of a refracting telefcope, agreeably to the 
experiments previoufly made on this fubjeft by Dr. Ro¬ 
bert Blair, as we have already Anted at p. 551. 
On the 26th of January, in the year 1799, Mr. Cater 
Rand, of Lewes in SuAex, took out a patent for “ an 
improved military and naval telefcope, for afeertaining 
dikances, and the fizeand extenfion of objefts, at fight, 
by means of a new micrometrical adjuflment.” This 
micrometrical telefcope, however, was nothing more 
than the parallel wire micrometer, applied to a common 
pocket achromatic telefcope, in which a vernier fcale pro- 
jefted from the eye-piece, and indicated the quantity of 
the meafured angle to the profefled accuracy of 6" ; but 
how the inftrument was kept Aeady enough without 
a Aand for the ufe of fuch a micrometer, is not ex¬ 
plained. 
Mr. Dudley Adams, of Fldet-Areet, optician, took out 
a patent, on May 30, 1800, for rendering telefcopes more 
portable ; the objed of which was to fecure the advantage 
to be derived from ufing tubes, with flits made in fuch a 
way as to make them move fmoothly, and yet without 
ihake, within one another. 
Vot. XVII. No. laoi. 
ICS. G03 
Mr. G. H. Brown, fecretary to the WeAminffer fire- 
office, in Bedford-Areet, Covent-Garden, has deferibed, 
in the nth volume of the Repertory of Arts and Manu¬ 
factures, a reflecting telefcope, that always lies in an ho¬ 
rizontal pofition ; and, receiving the rays of light on an 
inclined plain mirror, having a central perforation, and 
placed near the infertion of the eye-tube, reflects them to 
the large concave fpeculum, which, by a fecond reflection 
forms the image in the eye-tube. Benjamin Martin con- 
AruCted a reflecting telefcope in this way, which he ul'ed 
in a vertical pofition for terreftrial objects ; and the only 
difference in the two conffruCtions feems to be, that in 
Martin’s the main tube was reclined when viewing ele¬ 
vated objects, fuch as the heavenly bodies, whereas 
Brown’s plain mirror has a vertical motion independently 
of the main tube. They have neither of them come into 
common ufe. v ' 
Mr. Manton,gunfmith, of Davis-Areet, Berkeley-fquare, 
London, took out a patent on the 23d of January, 1810, 
for fecuring the ufe of an exhaulted tube, on a fuppofition 
that there would be more light when the rays were refract¬ 
ed to a focus in vacuo. 
Mr. Cornelius Varley, artiA, now of Tottenham-court- 
road, London, took out a patent for a graphic telefcope, for 
the purpofe of delineating drawings from nature, on the 
principle of Dr. WollaAon’s camera lucida, the date of 
which is April 5, 1811. And on the 21A of May of the 
fame year, Dr. BrewAer of Edinburgh, and Mr. Harris,, 
optician, of Holborn, London, jointly took out a patent 
for a micrometrical, double-image, and coming-up, glafs, 
&c. which has its fcale of meafurement running longitu¬ 
dinally along the tube. 
Jean Rondoni, of Oxford-Areet, gent, took out a patent 
on the 20th of January, 1815, “ for certain improvements 
in the conAruCtion of refracting telefcopes, communi¬ 
cated to him by a certain foreigner rending abroad.” M. 
Rondoni declares the invention to confiff in varying the 
magnifying power of dioptric telefcopes. When theeye- 
glafles are to be concave, or bear a diminiffiing power, he 
forms the eye-part of two concave glafles, placing one 
in the fide or tube neareA the eye, and the other in the 
next, or fecond, or third, or other tube, from the eye, as 
the power of the glafles or other circumAances may re¬ 
quire ; by which means he is enabled to bring near, toge¬ 
ther, or divide apart, the faid two concave glafles; and, 
as the fame are brought nearer together, the magnifying, 
power increafes, and, as they are divided apart, the con¬ 
trary. With regard to varying the magnifying power of 
telefcopes of the ufual conAru&ion, \yich four convex 
glafles in the eye-part, he conArudts them in fuch manner, 
that the third and fourth glafles from the eye may be 
moved farther from, or nearer to, the eye. The means 
of eft'efling this are fully deferibed with figures in the lpe- 
cification ; which we do not think it neceffary to copy in 
this place ; but refer to the Rep. of Arts, vol xxvii. N. S. 
We have next to fpeak of the conAruftion, orfittin°-- 
up as it is called, of telefcopes; that is, the fixing them 
on a Aand in the firmeA and moA favourable manner for 
making oblervations. 
Now that the long aerial telefcopes are no longer in ufe, 
we fliall not fill our pages by defending the different kinds 
of mechanifm that were applied for rendering them ufe- 
ful in obfervations; by Huygens, Perrault, SebaAian, 
Mairan, and others ; moff of which are deferibed in vols. 
j. v. and vi. of the “Machines approuvees par l’Acade- 
mie, &c.” to which we refer the curious reader who 
willies to know the particulars. All the fupports for long 
telefcopes had neceflarily one property, which is defina¬ 
ble alfo in Aands that are made for modern telefcopes, 
but which is frequently neglefled ; and that is, the oL- 
jeft-end of the telefcope was Aeadily fupported by fome 
point of re A .near the remote extremity, where the rays 
were incident. Indeed all forms where the tube is fup- 
porte-d only near the middle, or where the whole imme- 
7 F diaitely, 
