COO OPT 
We may alfo mention Francis Fontana, an Italian, as one 
who claimed the honour of this invention in the year 
1608; but from what we have already faid, of Janfen par¬ 
ticularly, he cannot be confidered as the firft inventor, 
though it is pofiible that the report of fuch an invention 
having taken place might incite him to devife the means 
of effecting a funilar contrivance'. This, it is generally 
underltood, was the cafe with the famous Galileo, who, 
when profeffor of mathematics at Padua, heard it reported 
at Venice, in the year 1609, that a Dutchman had pre- 
fented count Maurice of Naffau with an optical inftru- 
rnent, which had the property of making diftant objeds 
appear as though they were near; but, notwithftanding 
about twenty years had elapfed fince the invention, the 
means ufed for producing the wonderful effect were not 
known ; and Galileo, on his return to Padua, in a very 
few days not only contrived but conftruded a telefcope, 
which be prefented to the doge Leonardo Donati, and to 
the fenate of Venice, together with an account of the 
conftrudion and ufes which the inftrument might be ap¬ 
plied to, both by fea and land 5 for which fervice it is well 
known that his ftipend as profeffor was henceforth tripled. 
Among other dilcoveries that were made with Galileo’s 
telefcope in the heavenly regions, the four fatellites of 
Jupiter were found by him to revolve round this planet 
in their refpedive periods, and were called the Medicean 
/tars, in honour of the houfe of Medici. This difcovery 
took place early in the year 1610 ; and Galileo, purfuing 
his favourite ftudy till March, publifhed at Venice his 
“ Nuncius Sidereus,” containing an account of all his 
dilcoveries, and dedicated it to Coftno, the grand duke 
of Tufcany, who, in a letter written by himfelf, invited 
the aftronomer to quit Padua for an increafed ftipend, 
without the labour of a ledurefhip. The firft telefcope 
which Galileo conftruded had only a power of three 
times ; his fecond was fix times more powerful; and his 
third magnified thirty-three times, which, at fo early a 
-date, was no contemptible inftrument. Hence Galileo, 
though evidently not the firft maker of a telefcope, has 
been confidered as entitled to all the merit that is due to 
fuch a noble invention,feeing he had no model before him, 
nor inftrudions how to proceed in the accomplifhment of 
his ingenious work. 
Bur, though Galileo was fuccefsful in the conftrudion 
and ufes Of his telefcope, which was of the refrading 
fort, with a concave eye-glafs., yet it remained for that 
fagacious mathematician Kapler to explain, on philofophi- 
cal principles, the rationale of that conftrudion. It was 
he who firft explained the nature and effects of both the 
converging and diverging rays of light, after pafling 
through the.refpedive lenfes, and who demonftrated the 
principles by which new arrangements might be made in 
the glafl'es, that would^produce a fuperior inftrument. He 
fhowed that in fmall obliquities of incidence, the angle of 
incidence exceeds the angle of refraction about three 
times. He alfo firft proved, that in a plano-convex lens, 
parallel rays are made to converge to a point which is 
• diftant from the lens juft the diameter of the fphere of 
convexityj and that, if both fidesxff the lens are equally 
convex, this point will be at the centre of the circle of 
convexity. It remained, however, for Cavallieri to dif- 
cover and to prove, in cafes where the radii of curvature 
of the two tides of a double-convex lens are unequal, 
that, as the fum of both the diameters is to one of them, 
fo is the other to thediftance of the focus ; and it may be 
proper to notice here, that the fame rules are applicable 
to concave lenfes, except that the focus is at the contrary 
■fide of the glafs. 
It is remarkable, hou'ever, that Des Cartes, thepupil of 
Kepler, makes no mention of his tutor’s improvements 
in the art of conftruding a telefcope having been car¬ 
ried into execution for feveral years after Galileo’s was 
brought into ufe. It was not till the year 1630, that 
Scheiner defcribes, in his “ Rofa Urfina,” the plan of 
fubftituting a convex infleadof a concave eye-glafs, as 
ICS. 
fuggefted by Kepler, to be ufed for nftronomical purpofes, 
where the iqverfion of the objed is a matter of no im¬ 
portance, but where the increafed field of view is of ma¬ 
terial cor.fequence. The fame mechanician foon after 
added a fecond convex glafs to his eye-tube, by means of 
which the objeds become ered, which addition w'as no 
improvement to the vifion, but rather a detriment ; and 
after him, Rheita gave an ered pofition to objects, by 
ufing three ftmilar lenfes in the eye-tube inftead of two, 
which greatly improved the vifion, without other detri¬ 
ment than the lofs of a little light: and, becaufe Rheita’s 
telefcope was adapted for viewing objects on the earth, 
as well as in the heavens, it was dillingUiHied by the 
name of the terre/rial telefcope, by way of diilindion 
from Scheiner’s a/ronomicail one. In both thefe tele- 
fcopes, as well as in Galileo’s with a concave eye-glafs, 
the power is eftimated from the focus of the objed-glafs 
divided by the focus of the eye-glafs. 
The ftudy of dioptrics row became general, and feve¬ 
ral improvements were offered by different individuals in 
the conftrudion of the refrading telefcope ; but among 
the real improvers muff be placed the very ingenious 
Huygens, who, being well acquainted with the aberra¬ 
tion of the rays of light arifing from the fpherical figurs 
of the glaffes, contrived a better arrangement of the eye- 
glaffesthan had before been devifed. It was however very 
foon found, that the power of a telefcope of any of the 
preceding conftrudions, could not be increafed by Short¬ 
ening the focus of the eye-glafs alone, beyond certain 
limits, without introducing great indiftindnefs, arifing 
from the fpherical aberrations ; and that the belt mode 
of gaining power, without diminution of light and dif- 
tindnefs, is an increafe of the focal length, without 
much increafe of aperture of the objed-gla-fs ; and a 
little experience fhowed, that it is neceffary to increafe 
this length in the duplicate ratio of the propofed increafe 
of power: i.e. in order to magnify twice as much, the 
focus of the objed-glafs rnuft be made four times as long 
as that of another telefcope that has the fame light and 
diftindnefs ; and for any other power in a fimilar propor¬ 
tion. The confequence of this difcovery was, that dif¬ 
ferent makers began to vie with each other, with refped 
fimply to the length of their telefcopes : among thefe 
may be mentioned, F.uftatio Divini at Rome ; Campani 
at Bologna ; fir Paul Neille, Mr. Reive, and Mr. Cox, 
in England ; and in France, Borelli and Auzout. The 
laft-mentioned mechanician fucceeded in grinding an ob¬ 
jed-glafs of the aftonifhing length of 600 feet; and, it 
is faid, that Hartfocker made them even longer than this. 
It will here occur to the reader, that tubes of this 
enormous length, if pradicable, could not be manage¬ 
able by an obferver; and hence we find, that thefe very 
long objed-glafies were fixed on the top of long poles, 
or to growing trees, and fo contrived as to be capable of 
adjuftment for the axis of vifion when turned to different 
altitudes, agreeably to the required pofition of the remote 
eye-glafs. 
But, while the length of the telefcope was thus in¬ 
conveniently increafed, and the trouble of making good 
obfervations therewith proportionably augmented, it be¬ 
came a queftion to determine in what proportion the 
aperture might be enlarged with the increafe of focal 
length of the objed-glafs. Auzout wrote a paper, and 
-deliveredfit to the Royal Society in the year 1665, in 
which he affirmed,- that the diameter of the objed-glafis 
ought always to be in a fub-duplicate ratio of its focal 
length, or nearly fo ; and accordingly drew up a table of 
apertures fuitable for all focal lengths, from 4 inches to 
400 feet: upon which Dr. Hooke very properly remarked, 
that the fame glafs may have its aperture advantageoufly 
enlarged or diminifhed, according to the quantity of light 
proceeding from the objed viewed. 
While powerful telefcopes were thus obliged to be un¬ 
manageably long, and obtained the name of aerial telgr 
fcopes, from the circumftanc# of their having no tub^‘ 
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