€02 . OPT 
reprefenting the objeCt at the point of eroding is only 
viewed ; which circumftance conllitutes the eflential dif¬ 
ference in the two conllru&ions ; and it is very probable 
that the light proceeding from the image of an object may 
not be fo vivid as that proceeding from the object itfelf, 
of which the image may be confidered as a let's enlightened 
reprefentation. See the Edinburgh Review for Nov. 1814. 
It is remarkable, that no improvement was made on 
Newton’s fmall telelcopes till about the year 1723, when 
Hadley prel'enfed to the Royal Society a reflecting tele- 
fcope of Newton’s conftrudtion, in which the focus of the 
I'peculum was 10 feet 55 inches. Though Newton’s were 
only fix inches long each, the}' were compared to the fix- 
feet refraCtors, fuch as were made at that time; but what 
mult have been the public feeling, when Hadley produced 
his enlarged inltrument! This was found at lead equal 
in power to the famous Huygeuian refradtor of 123 feet; 
at lead, its power and didindtnefs were equal, though the 
light was not quite lb bright. 
Excepting the manner of making the fpecula, we have, 
in the Philofophical Tranfadtions of 1723, a complete de- 
Icription, with a figure, of this telefcope, together with 
that of the machine for moving it; but, by a drange 
omiffion, Newton’s name is not once mentioned in that 
paper; fo that any perfon not acquainted with the hidory 
of the invention, and reading that account only, might 
be apt to conclude that Hadley had been the foie inventor. 
The fame celebrated artid, after finiffiing two telelcopes 
of the Newtonian condrubtion, accomplilhed a third, of 
the Gregorian form ; but, it would feem, lefs fuccefsfully. 
Mr. Hadley (pared no pains to indrubt Mr, Molyneux 
and the Rev. Dr. Bradley; and, when thofe gentlemen had 
made a fufficient proficiency in the art, being delirous that 
thefe telelcopes lliould become more public, they libe¬ 
rally communicated to fome of the principal indrument- 
makers of London the knowledge they had acquired from 
it i m. 
'Mr. James Short, as early as the year 1734, had figna- 
lized himfelf at Edinburgh by the excellence of his tele- 
fcopes. Mr. Maclaurin wrote that year to Dr. Jurin, 
“ that Mr. Short, who had begun with making glafs fpe¬ 
cula, was then applying himfelf to improve the metallic; 
and that, by taking care of the figure, he was enabled to 
give them larger apertures than others had done ; and that, 
upon the whole, they furpalfed in perfedlion all that he 
had feen of other workmen.” He added, “ that Mr. 
Short’s telelcopes were all of the Gregorian condrublion ; 
and that he had much improved that excellent invention.” 
This charadler of excellence Mr. Short maintained to the 
lad ; and with the more facility, as he was well acquainted 
with the theory of optics. It was fuppofed that he had 
fallen upon a method of giving the parabolic figure to his 
great I'peculum ; a point of perfedlion that Gregory and 
Newton had del'paired of attaining; and that Hadley had 
never, as far as we know, attempted. Mr. Short indeed 
affirmed, that he had acquired that faculty, but n'ever 
would tell by wdiat peculiar means he elfebted it; fo that 
the fecret of working that configuration, w hatever it was, 
died with that ingenious artid. 
Mr. Mudge, however, has lately realized the expedi¬ 
tion of fir Ilaac Newton, who, above a hundred years 
ago, prefaged that “ the public would one day pollefs a 
parabolic Ipeculum, not accomplilhed by mathematical 
pules, but by mechanical devices.” This was a defulera- 
tum, but it was not the only want fuppiied by this gentle¬ 
man : he has taught us likewile a better compolition of 
metals for the fpecula, how to grind them better, and how 
to give them a finer polilli ; and this lad part, (namely, 
the polilh,) he remarks, was the moll difficult and effen- 
tial of the whole operation. “ In a word (lays fir John 
Pringle), I am of opinion, there is no optician in this 
great city (which hath been fo long and fo judly renowned 
for ingenious and dexterous makers of every kind of ma¬ 
thematical inllruments) fo partial to his own abilities as 
not to acknowledge, that Mr. Mudge has opened to them 
ICS. 
all fome new and important lights, and has greatly Im¬ 
proved the art of making refledting telefcopes.” 
The late reverend and ingenious John Edwards devoted 
much of his time to the improvement of refledting tele¬ 
fcopes, and brought them to fuch perfedlion, that the 
late Dr. Malkelyne, adronomer-royal, found telefcopes 
conftrutted by him to furpafs in brightnefs, and other re- 
fpedls, thofe of the lame fize made by the bell artills in 
London. The chief excellence of his telefcopes arifes 
from the compofition, which, from various trials on me¬ 
tals and femi-metals, he dilcovered for the fpecula, and 
from the true parabolic figure, which, by long pradtice, he 
had found a method of giving them, preferable to -any that 
was known before him. His diredtions for the compofi¬ 
tion of fpecula, and for calling, grinding, and polilhing, 
them, were publilhed, by order of the commiffioners of 
longitude, at the end of the Nautical Almanac for the year 
1787. To the fame almanac is alfo annexed his account 
of the caufe and cure of the tremors which particularly 
affedl refledting telefcopes more than refradling ones; to¬ 
gether with remarks on thefe tremors by Dr. Mafkelyne. 
To Mr. Short we are indebted alfo for the excellent 
contrivance of an equatorial telefcope, or, as he likewife 
called it, a portable obfervatory; for, with it, pretty accu¬ 
rate obfervations may be made with very little trouble, by 
thofe who have no building adapted to the purpofe. The 
inltrument confilts of a piece of machinery, by which a 
telefcope mounted upon it may be diredted to any degree 
of right alcenfion or declination ; fo that, the place of any 
heavenly bodies being known, they may be found without 
any trouble, even in the day-time. As it is made to turn 
parallel to the equator, any o'ojedt is ealily kept in view, 
or recovered, without moving the eye'from its fituation. 
By this inllrument, moll of the liars of the firll and lecond 
magnitude have been feen even at mid-day, when the fun 
was lliining bright; as alfo Mercury, Venus, and Jupiter. 
Saturn and Mars are not fo ealy to be feen, on account of 
the faintnefs of their light, except when the fun is but a 
few hours above the horizon. This particular ellebl de¬ 
pends upon the telefcope excluding almolt all the light, 
except what comes from theobjedl itfelf, and which might 
otherwife efface the impreffion made by its weaker light 
upon the eye. Any telefcope of the fame magnifying 
power would have the fame efteCt, could we be lure of 
pointing it. right. Mr. Ramfden invented a portable or 
equatorial telefcope, which may perhaps fuperfede the life 
of Mr. Short’s. 
The greatell improvement in refracting telefcopes hi¬ 
therto made public, is that of Mr. Dollond, of which an 
account has already been given in a preceding part of this 
article, in which his difeoveries in the fcience of optics 
were explained. But, belides the obligation weare under 
to him for correcting the aberration of the rays of light 
in the focus of objeCt-glafles, (lee Achromatic, vol. i.) 
lie made another conliderable improvement in telefcopes, 
viz. by correcting, in a great meafure, both this kind of 
aberration, and alfo that which arifes from the fpherical 
form of lenfes, by an expedient of a very different nature, 
viz. increajiug the number of eye-g/u/j'es. 
“ It any perfon (lays he) would have the vifual angle 
of a telelcope to contain 20 degrees, the extreme pencils 
of the field mull be bent or refraCted in an angle of 10 
degrees; which, if it be performed by one eye-glafs, will 
caule an aberration from the figure, in proportion to the 
cube of that angle ; but, if two glalfes be fo proportioned 
and fituated, as that the refraction may be equally divided 
between them, they will each of them produce a refraClion 
equal to half the required angle; ami therefore, the aber¬ 
ration being proportional to the cube of half the angle 
taken twice over, will be but a fourth part of that which 
is in proportion to the cube of the whole angle; becaufe, 
twice the cube of 1 is but£ of the cube of 2 ; fo the aber¬ 
ration from the figure, where two eye-glades are rightly 
proportioned, is but a fourth of whatithnill unavoidably 
be where the whole is performed by a fingle eye-glafs. 
