powers. 
NATURE 
189 
THURSDAY, JULY 1, 1886 
KEPLER’S CORRESPONDENCE WITH 
HERWART VON HOHENBURG 
Ungedruckte wissenschaftliche Correspondenz zwischen 
Tohann Kepler und Herwart von Hohenburg, 1599. 
Erganzung zu: Kepleri Opera Omnia, ed. Chr. Frisch. 
Nach den MSS. zu Miinchen und Pulkowa edirt von 
C. Anschiitz. (Prag: Victor Dietz, 1886.) 
ERWART VON HOHENBURG was a highly 
stimulating correspondent. His scientific curiosity 
was insatiable ; his official duties as Bavarian Chancellor 
precluded personal research ; and he accordingly deputed 
to the busy brain of Kepler the working out of problems 
which engaged his scanty leisure, while baffling his 
The pressure of his demands was, indeed, so 
severe that Kepler at times bewailed himself in con- 
fidential quarters over the grinding labours they imposed 
upon him : but he could ill afford to quarrel with a patron 
who was as generous as he was inquisitive ; and he thus 
continued to evolve for his benefit the stores of curious 
learning and adventurous theory of which some consider- 
able specimens have lately been unearthed, and are now 
presented to the public. 
The correspondence took its origin from the publica- 
tion, in 1596, of the “ Mysterium Cosmographicum,” by 
which Herwart’s admiring attention was drawn to the 
speculative young “ mathematicus” of the Styrian States; 
and continued from October 1597 to December 1600. | 
There was, however, a gap in its records. Three letters, 
known to have been written by Kepler to Herwart in the | 
year 1599, were not forthcoming. Dr. Frisch, the late 
indefatigable editor of Kepler’s ‘‘ Opera Omnia,” gave up 
the search as hopeless ; and the detection of the latitant 
documents became possible only with the correction, in a 
the Munich State Library, of an error in the old printed 
one,—an example, were such needed, of the uses to 
historical research of the least inviting bibliothecarian 
drudgery. The opportunity for discovery was promptly 
turned to account by M. Carl Anschiitz, the editor of the 
present brochure ; who deserves the acknowledgments of 
every one interested in scrutinising the workings of a 
most singularly and brilliantly constituted mind, not alone 
for the: zeal of his inquiries, but also for the elaborate 
care with which he has set forth their results, rendering 
intelligible by copious annotations what must otherwise 
have remained, to the vast majority of readers, hopelessly 
obscure. 
The truth chiefly emphasised by a perusal of these 
remarkable letters is that of the inextricable entanglement 
of Kepler’s mystical with his scientific views. Many men 
have speculated wildly while investigating acutely ; Kepler 
alone, perhaps, investigated acutely decawse he speculated 
wildly. His visions of abstract beauty and order in a 
neatly fenced and finished universe warmed his fancy, 
and inspired and lightened labours which would otherwise 
have been insupportable. His discoveries were the fruit 
of his illusions, because his illusions were faithfully and 
unwearyingly confronted with the realities of nature. He 
VOL. XXxIv.— No. 870 
was a.dreamer; but he was not content to leave his 
dreams undisturbed by facts. Hence his superiority— 
” ? ’ , ‘ b ed + 
eEoxos ’Apyeiwy ketbadny Te Kal evpeas @jLovs— 
to the common run of Pythagorean enthusiasts, and hence 
his great name in scientific history. 
The topics discussed in the present correspondence 
forcibly illustrate the compound nature of his mind, no 
less visionary in its instincts than positive in its methods. 
They include the theory of eclipses, the vatzonale of 
planetary influences, the harmonic relations of planetary 
velocities, the date of the birth and the horoscope of 
Augustus, the nature of terrestrial magnetisn, and the 
position, actual and primitive, of the north magnetic 
pole. The first of the three letters is dated from Gratz, 
April 9 and 10, 1599. It opens with a pompous eulogium 
on Tycho Brahe. “Taceant omnes, et Tychoni Brahe 
Dani auscultent.” Nor does it detract, we are told, from 
his merits to have taken a wrong theoretical turn. His 
hostility to the motion of the earth nowise impairs the 
excellence of his observations and instruments. Each 
astronomer is free to embrace, without discredit to his 
skill and erudition, whatever “religion of movement ” 
seems best to himself. ‘Sed ad rem.” 
Tycho, deceived no doubt by reports of coronal splen- 
dours (he had never himself witnessed the phenomenon), 
had denied the possibility of a total solar eclipse, the 
moon suffering, he alleged, a diminution of one-fifth of its 
apparent diameter when projected on the sun. Kepler, 
while unconvinced of the fact, was at no loss for an ex- 
planation. A dense lunar atmosphere, powerfully reflective 
of the sun’s rays, while partially permeable by them, was 
invoked by him to augment the seeming dimensions of 
the full moon, and throw a kind of subdued glory round 
the eclipsed sun. The perplexity started by Tycho was 
not, however, so easily allayed. !t kept cropping up at 
intervals ; and led eventually both to Kepler’s optical 
researches, and to what we may call his discovery of the 
new printed catalogue of the manuscript collections in | 
corona, as an actual fact to be reckoned with by science. 
The eclipse observed by Clavius at Rome in 1567 he 
showed to have been unquestionably total; the sun was 
fully coverea by the moon; yet an unlooked-for radiance 
survived (“ Op. Omnia,” t. il. p. 318). He accounted for 
it by the illumination of an “ethereal substance” in the 
solar neighbourhood, “not altogether nothing, but pos- 
sessing some measure of density”; nor have we yet got 
much beyond the approximate ratification of his con- 
jecture. 
Later in life Kepler formally laid down his arms before 
the lunar theory, after spending enormous labour on the 
effort to bring it into conformity with his Laws. But here, 
in these long-missing letters, he unexpectedly emerges 
as the discoverer of the moon’s annual equation. The 
fact seems to admit of no doubt; his words are explicit. 
The discrepancies between the observed and calculated 
times of eclipses compelled the correction. Had not 
Copernicus, he remarks, been occupied with greater 
things, he must have introduced the same “ annual in- 
equality ” depending upon the eccentricity of the earth’s 
orbit. ‘‘ What he neglected,” he adds, “I now do.” The 
chief merit of this important advance has usually been 
ascribed to Tycho. He had doubtless glimpses of its 
necessity, but omitted to follow them up. The earliest 
K 
