fiOl 
OPT 
to be inclofed in, the immortal Newton had his pene¬ 
trating- mind occupied with meditated improvements on 
the figure and arrangement of lenfes; and proceeding, as 
he always did, on rational principles, difcovered, from 
the elongated and coloured fpedtrum formed by rays of 
light paffing through a triangular prifm, and from expe¬ 
riments calculated to inveftigate the caufe of fuch an 
oblong form and coloured appearance, that light is not 
homogeneous, and that different rays are differently re¬ 
frangible when tranfmitted through the fame medium. 
This grand difcovery prefented difficulties Handing in 
the way of the improvement of the refradting or dioptric 
telefcope, apparently much greater than thole which had 
previoufly been difcovered as arifing only out of the fphe- 
rical figure of the glaffes ; and all hope of fuccefs in mak¬ 
ing fhort telefcopes of great power, and yet with fufficient 
light and diftindtnefs, but without an admixture of co¬ 
loured rays, was given up. 
Yet to a mind like Newton’s, it naturally occurred, 
that what could not be practically effedted by refradtion 
might probably be accomplifhed by ref left! on of the rays of 
light into a focus, where, as there would be no fepara- 
tion of the colorific rays by a refradting medium, there 
would be no colour nor elongation of the focal point, 
arifing from any other aberration than what might be 
caufed by the figure of the reflecting furface ; he therefore 
abandoned his propofed plan of grinding lenfes after the 
figure of fome of the conic fedtions, (for which fir Chrilto- 
pher Wren contrived a machine,) to avoid the effedts of 
fpherical aberration in dioptric telefcopes, and turned his 
mind to the improvement of catoptric or rather cato- 
dioptric telefcopes, which had been previoufly propofed 
to Des Cartes by Merfenne, and adtually conftrudted by 
James Gregory of Aberdeen. 
The firlt conftrudtion of the refledting-telefcope was the 
Gregorian, and molt of the portable refledtors continue 
to be of this conflrudfion at the prefent day ; its large 
fpeculum is concave, perforated at the centre, and placed 
at the interiorend of the large tube ; and the fmall refledtor 
is alfo concave, placed oppofite the central hole of the 
'large one, in fuch an adjuflable manner, that the rays, 
after a lecond reflection, crofs one another, and come to 
the eye-glafs in fuch a way, that an erctt pidture of the 
objedf, or rather of the image of the objedt, is formed on 
the retina of the eye. In this conftrudtion, it has been 
fuppofed that the figure of the large concave fpeculum 
ought to be truly parabolic, becaufe that is the figure re¬ 
commended by Newton for its conftrudtion ; but this 
conclufion is erroneous ; for it is the joint effedt of both 
the fpecula that muft be adverted to in their refpedtive 
figures, fo that the rays may come without aberration to 
the eye-glafs after both reflexions; and, in order to produce 
this joint effedt, the curve of the large fpeculum muft be 
fomewhat more than parabolic, viz. approaching to hy¬ 
perbolic, becaufe the fmall fpeculum is alfo concave, and 
has its feparate aberration. 
In the Newtonian conftrudtion, the large fpeculum is, 
or ought to be, truly parabolic, and the fmall one plane, 
fet diagonally at an"angle of 45 0 ; fo that the rays, after 
the fecond reflection, come to the eye-tube on the fule of 
the large tube, and near its aperture : the rays do not 
crofs here, but come to a focus at the eye-glafs, where the 
objedt is reprefented inverted and well defined, as well 
as bright; for, when the rays fall obliquely on the fmall 
refledtor, they are almoft all reflected without difperfion, 
■which is an advantage that this conftrudtion has over the 
Gregorian. When the Newtonian telefcope was propofed 
to Huygens, he had the candour to acknowledge, which 
proved to be the fadt, that there would not be that limit 
to the aperture of a refledtor that is prefcribed by natural 
neceffity to that of a refractor; and that the power, as well 
as light, may be made far to exceed thofe of the latter. 
The next conftrudtion of a reflecting telefcope was that 
of Caffegrain, defcribed in the Philofophic.nl Tranfadtions 
of the year 1672. This differs from the Gregorian only in 
VOL. XVII. NO. 1201. 
ICS. 
this particular, that the fmall fpeculum is convex, and the 
focus of the large or concave one may be longer than is 
required in the other, for the fame length of tube ; the 
rays do not crofs after the fecond reflection, and confe- 
quently the objedt is feen inverted, as in the Newtonian; 
but here the curve of the large fpeculum is lefs than pa¬ 
rabolic, in order that the joint effedt of both the reflec¬ 
tions may be an exemption from aberration. Thisadjuft- 
ment of the figures of the metallic furfaces is belt under- 
ltood and accomplifhed by the firft-rate opticians, and is 
but little known to mere theorifts. 
Of the Herfchelian telefcope w'e fliall only fay, in this 
place, that it differs from the Newtonian in no other re- 
fpedt except in its fize and powers, and that the fecond 
refledtor is difpenled with, the length of the tube being 
equal to the focal diftance of the large fpeculum, and the 
head of the obferver being confequently placed at the up¬ 
per end or aperture of the tube ; fo that, in this conftruc- 
tion, as little light as poflible is loft from the Angle reflec¬ 
tion, the principal lofs being that which is intercepted, 
on its entrance into the tube, by the head of the obferver. 
The parabolic curve for the face of the fpeculum is 
equally proper for the Herfchelian as for the Newtonian 
telefcope. 
From thefe fhort hiftorical notices it will be feen, that 
Merfenne firft fuggerted the hint for conftrudting a reflect¬ 
ing telefcope,'which muft have been before the year 1651, 
when his Catoptrics were printed; or, according to Des 
Cartes’s third and twenty-ninth letters, written in 1639, 
though not publifhed till 1666, before thefe letters were 
w’ritten. Gregory, who might or might not have feen 
Merfenne’s fuggeftion, publifhed an account of his con¬ 
ftrudtion in his Optica Promota, in the year 1663 ; but, as 
he was not a (kilful mechanic himfelf, it is underftood 
that his telefcope was but an indifferent one, and that the 
theory of his conftrudtion waas not completely realifed to 
his wifh. At this juncture, fir Ifaac Newton, who was a 
good mechanician, as well as mathematician and experi¬ 
mental philofopher, took the fubjedt into his confidera- 
tion; and, by his fuccefsful labours, prevented the in¬ 
vention from falling into oblivion. His proceedings met 
with interruption from the occurrence of the plague; 
but, about the end of the year 1668, he began his experi¬ 
ments on fpeculum-metai, and, in the year 1672, pro¬ 
duced two fmall refledling telefcopes. In thefe, the large 
fpecula were ground into a fpherical concave furface, as 
being the eafieft to execute; but he was aware that the 
parabolic curve, recommended by Gregory, would be pre¬ 
ferable, “when it could be accomplifhed by mechanical 
contrivances,” which he judged to be within the reach of 
human ingenuity. The refult of thefe labours was com¬ 
municated to the Royal Society of London ; and, through 
the medium of their fecretary, Mr. Oldenburgh, to the 
ingenious Huygens, whoteftified his approval of this con¬ 
ftrudtion in an account which was publifhed in the Journal 
des Sqavans for the year 1672 ; and, in this way, nearly 
the whole of Europe became acquainted with the Newto¬ 
nian conftrudtion. In the mean time Caffegrain, a French¬ 
man, who had varied Gregory’s conftrudtion, by fubfti- 
tuting a convex inftead of a concave fmall fpeculum, as we 
have already lta ted, in the fame journal (des Sgavans, 1672) 
contefted the honour of having been the firft improver of 
the original Gregorian telefcope; which claim drew' from 
Newton feveral objections to Caffegrain’s conftrudtion, 
that will indeed apply equally to the Gregorian. We 
have however recently witneffed, in captain Kater, an 
advocate for Caflegrain’s telefcope, in preference to that 
of Gregory, (fee Phil. Tranf. of London, 1813 and 1814.) 
principally with refpedt to the brightnefs and diltindtnefs 
of objedts refpedtively feen by them, even in the propor¬ 
tion of 6 to 3 - 3. His conclufion is, that much of the light 
is diffipated by the crojfing of the rays in the focal point; 
which isadottrine waiting for confirmation. In Caffe- 
grain’s telefcope, the pidture of the objedt itfelf is viewed 
by the eye; but, in Gregory’s, the pidture of the image 
7 O reprefenting 
