92 HISTORY OF SCIENCE. 



the hypothesis which led to results discordant with the facts. Before 

 arriving at the discovery of the true nature of the orbit of Mars, Kepler 

 thus tried no fewer than nineteen other paths. The twentieth guess 

 was right ! The appearances were in agreement with the supposition, 

 and Kepler announced that the orbit of Mars was an ellipse, thus 

 overthrowing the ancient prejudice concerning circular motions which 

 had so long checked the progress of astronomical science, and pre- 

 paring the way for the grand discoveries of Newton. 



At length he obtained the arrears of his salary, and in 1613 he was 

 appointed to a professorship of mathematics at Lintz, where he passed 

 sixteen years of his life, and where he published in 1619 a year me- 

 morable in the history of science a work entitled "The Harmonies of 

 the World," containing the announcement of the third of his celebrated 

 laws,, that, namely, connecting the distance of a planet from the sun 

 with the period of its revolution. This law is expressed mathematically 

 by saying that the squares of the times of the revolutions are propor- 

 tional to the cubes of the distances. About the same time Kepler pub- 

 lished a work entitled " An Epitome of the Copernican Astronomy/'' 

 This book contains his opinions and reasonings in astronomy presented 

 in the popular form of question and answer. He puts forward an ex- 

 planation of the law of periodic times ; but of the four suppositions 

 he makes, three are now known to be false. Very curious is an argu- 

 ment he advances against the notion then prevalent that each planet 

 is directed in its movements, or carried round, by an angel. " In that 

 case," says Kepler, "the orbits would be perfectly circular; but the 

 elliptic form which we find in them rather smacks of the lever and 

 material necessity." 



BAPTISTA PORTA (1545 1615) was a native of Naples, where he 

 distinguished himself at a very early age by publishing his book on 

 Natural Magic. This work describes a great number of curious ob- 

 servations, experiments, and contrivances, and although many of these 

 are destitute of any scientific value, the whole work nevertheless shows 

 that its author was acquainted with the true principles of science. The 

 book being put forth at the period when the art of printing was afford- 

 ing unprecedented facilities for the dissemination of knowledge, be 

 came very popular, and was soon translated into the principal Euro- 

 pean languages. Its wide circulation may also be taken as an evidence 

 of an awakening interest in all things pertaining to the truths of nature. 

 Porta also gathered round himself at Naples a circle of personal friends 

 devoted to pursuits like his own, and this little circle of curious and 

 learned persons constituted itself into a scientific society meeting to 

 discuss and hear the accounts of new experiments. These proceedings 

 were regarded by the ecclesiastics with no little jealousy, but Porta and 

 his associates were too circumspect to afford the Church any pretence 

 for persecution. 



Porta's name must be associated with the invention of the camera 



