680 
KEP 
Scripts befides. Han fell of Leipzig, into whofe bands 
they fell, announced in 1714, a complete collection of 
Kepler’s works in twenty-two volumes, which was de¬ 
signed to comprehend thefe manuferipts; but, for want 
of fufficient fupport, this projeCt was never executed. 
The public is, however, indebted to him for the corre- 
fpondence between Kepler and the moll eminent Scholars 
of his,time, which is of the higheft importance for the 
hiftory of his life. This collection of letters, the publi¬ 
cation of which was encouraged by the imperial court, 
appeared elegantly printed in 1718 under the title of, 
EpiJloLz ad Joannem Keplerum Mathematician. Ctefareutn fcr'tp- 
ta , infertis ad eafdem refponjiojiibus Keblerianis, quot quo hade- 
tius reperiri potuerunt. Kepler’s manuferipts were after¬ 
wards pledged at Frankfort on the Mayn by Hanfch, w ho 
died in 1749 at Vienna in neCeffitous circumltances, and 
at length purcliafed by the Ruffian government and con¬ 
veyed to Petersburg, where Euler and fome others were 
commiffioned to examine them, and feleCl fuch as they 
deemed mod: worthy for publication. 
Kepler was not Satisfied with obferving the changes of 
the earth and the other planets; he was anxious to discover 
the primary caufe of thefe changes; and therefore aifumed 
that all the itars, the fixed (tars as well as the planets, had a 
foul,and that they were furnifhed with amultitudeof nerves 
and fibres, which fet this foul in motion for the purpofe 
of effecting all thefe changes. For this conjecture he has 
been feverely cenfured by fome, efpecially by Schookius, a 
Dutch writer; and yet it is probable, that by the foul he 
did not intend to denote a Spiritual effence, but rather a 
primary phyfical principle, bearing fome analogy to the 
vital principle discovered by Defcartes in animals, and 
carefully diftinguifhed by him from the foul, or to Gal-- 
fendi’s foul of brutes. Kepler had a particular pallion 
for finding analogies and harmonies in nature, after the 
manner of the Pythagoreans and Platonifts; and to this 
dil'pofition w'e owe fuch valuable difeoveries as are more 
than fufficient to excufe his conceits. Three things, he 
tells us, he anxioufly fought to find the reafon of from 
his early youth : Why the planets were fix in number ? 
Why the dimenfions of their orbits were fuch as Coper¬ 
nicus had deferibed from observations ? and, What was 
the analogy or law of their revolutions? He fought for 
the reafons of the firft tw'o of thefe in the properties of 
numhers and plane figures, without fuccefs. But at length 
refieCf ing, that, while the plane regular figures may be infi¬ 
nite in number, the ordinate and regular Solids are five only, 
as Euclid had long ago demonllrated ; he imagined that cer¬ 
tain mylteries in nature might correfpond with this remark¬ 
able limitation inherent in the effences of things; the rather 
ashefound thePythagofeans had made great uSe of thofe five 
regular Solids in their philefophy. He therefore endeavoured 
to find fome relation between the dimenfions of thofe folids 
and the intervals of the planetary Spheres; and, imagining 
that a cube inferibed in the fphe;_ of Saturn would touch 
by its fix planes the Sphere of Jupiter, and that the other 
four regular folids in like manner fitted the intervals that 
are betwixt the Spheres of the other planets, he became per¬ 
suaded that this was the true reafon why the primary planets 
were precifely fix in number, and that the Author of the 
world had determined their difiances from the fun, the 
jtentre of the fyftem, from a regard to this analogy. Be¬ 
ing thus poflelled, as he thought, of the grand Secret of 
the Pythagoreans, and being mightily pleafed with his 
difeovery, he published it in 1596, under the title of Myjle- 
rium Cofmographicum ; and his partiality for this book is 
eafily accounted for by his predileCfion for that hypo- 
thefis. 
In phyfics Kepler did not, like the moderns, confider 
heat as a peculiar and extremely Subtle lubftance, which 
eafily penetrates bodies and is communicable by one to 
another, but as a mere quality. He was the firlt that ob¬ 
served the hexagonal cryffallization of the flakes of Snow ; 
and in a diftinCt work called the attention of natural phi¬ 
losophers to the Subject. He was one of the firlt that af- 
L E R. 
cribed the motion of the fea, termed ebb and flood, to 
the preflure of the moon upon the earth, (which Stevin 
maintained before him ;) and very juftly remarked, that 
the attraction of the moon would draw the whole ocean 
to that luminary, were it not prevented by the attraction 
of the earth. It were to be wifiied that he had entered 
in fome of his works into a more extenfive and minute 
investigation of this important fubjeCh He made many 
observations on the refraction of the rays of light in wa¬ 
ter and glafs, and fought to determine with precision the 
angle of this refraCtion. This led him Still farther, and 
he affiduoufly Strove to find the aftronomical refraction 
for every degree of latitude, one of the molt difficult talks 
that the human underftanding ever impofed on itfelf. If it 
cannot be denied that his labours paved the way for fu¬ 
ture difeoveries; yet he committed one great error, in af- 
ligning to the air an equal degree of denfity to the ex¬ 
treme limits of the atmofphere, though barometrical and 
other observations demonstrate that in the higher regions of 
the atmofphere the air is more rare than in the lower, where 
it is rendered denfer by the Superincumbent weight. As 
the dimenfions of the angle of the refraction of light de¬ 
pend on the denfity of the matter in which it is refraCt- 
ed, Kepler’s inquiries could not lead to any Satisfactory 
refults. 
His obfervations on the eye and the function of fight, 
furpafs in accuracy thofe of all his predecelfors. Before 
his time it was the received opinion, that from each point 
of a luminous body, only one Single ray reached the eye. 
He, on the other hand, truly afierted, that from every point 
of the vifible objeCt, after the reflection of the rays of light, 
pencils of rays reach the eye ; and that an objeCt cannot 
be distinctly Seen unlefs each of thefe pencils of rays, af¬ 
ter its refraClion in the cryftalline humour, is again united 
in a point on the retina, upon which the figure of the ob- 
jeCt is accordingly painted. From thefe preliminaries he 
deduced the real caufe why far-iighted perfons cannot fee 
objects diftinCtly at a certain difiance without concave 
glafles, and near-lighted perfons without fuch as are con¬ 
vex. He made many other difeoveries of this kind ; fo 
that he not only very much improved and extended the 
knowledge of optics, but was the founder of an entirely 
new Science, that of dioptrics, which Since his inquiries 
has been considered as a branch of the mathematics. He 
was likewife the inventor of the aftronomical telefcope, 
though he did not himfelf carry his theory into execution. 
Under his directions, his contemporary, father Scheiner, 
constructed one of thefe instruments, and found it ex¬ 
tremely Serviceable in his obfervation of the folar Spots. 
Highly as thefe difeoveries and improvements may be 
calculated to tranfmit Kepler’s name to the remotelt pos¬ 
terity, his principal merits confifted in the fervices which 
he rendered to Astronomy. See that article, vol. ii. 
p. 337, 42+. By him the Copernican fyftem was fo firmly 
established, that its victory over the Syftems of Ptolemy 
and Tycho Brahe was no longer doubtful; and, fince 
he communicated his ideas to the world, no objection of 
any weight has been alleged againft it. It is a pleafing 
ipeCtacle to obServe how all that is good and true univer¬ 
sally gains ground at laft among mankind ; for, though 
none of the above-mentioned three fyltems can be proved 
to complete demonstration, and we are as yet permitted 
to enjoy but a glimpfe of the great plan of the univerfe; 
yet among conjectures fome are more diftinguilhed by 
truth and by their coincidence with the general mafs of 
what we know than others. In Kepler’s time people 
univerfally believed, with Ptolemy of old, that the earth 
was a fixed Star, round which the fun and all the other 
planets revolved ; and, if Copernicus declared the fun 
to be a fixed Star, and allotted to the earth a place among 
the planets, this arrangement was considered as little 
more than the Sport of a genius to which the praife of Sa¬ 
gacity could not be denied. But, when Galileo ferioufly 
endeavoured to give general currency to the notions of 
Copernicus, not only the church, which took under her 
proteCtioa. 
