96 ON THE MAGNITUDE OF THE SOLAR SYSTEM. 



shadow cone by noting the duration of lunar eclipses, and as the result 

 deduced from it happened to be nearly the same as that found by Aris- 

 tarchus, substantially his value of the parallax remained in vogue for 

 nearly two thousand years, and the discovery of the telescope was 

 required to reveal its erroneous character. Doubtless this persistency 

 was due to the extreme minuteness of the true parallax, which we now 

 know is far too small to have been visible upou the ancient instru- 

 ments, and thus the supposed measures of it were really nothing but 

 measures of their inaccuracy. 



The telescope was first jioiuted to the heavens by Galileo in 1009, but 

 it needed a micrometer to convert it into an accurate measuring instru- 

 ment, and tliat did not come into being until 1639, when it was invented 

 by William Gascoigue. After his death, in 1644, his original instru- 

 ment passed to Kichard Townley, who attached it to a 14-foot telescope 

 at his residence in Townley, Lancashire, England, where it was used 

 by Flamsteed in observing the diurnal parallax of Mars during its 

 opposition in 1672. A description of Gascoigne's micrometer was pub- 

 lished in the Philosophical Transactions in 1667, and a little before 

 that a similar instrument had been invented by Auzout, in France, but 

 observatories were fewer then than now, and, so far as 1 know, J. D. 

 Cassini was the only person beside Flamsteed who attempted to deter- 

 mine the solar parallax from that opposition of Mars. Foreseeing the 

 importance of the opportunity, he had llicher dispatched to Cayenne 

 some months i)reviously, and when the opposition came he effected two 

 determinations of the parallax 5 one being by the diurnal method, from 

 his own observ^atious in Paris, and the other by the meridian method, 

 from observations in France by himself, Komer, and Picard, combined 

 with those of llicher at Cayenne. This was the transition from the 

 ancient instruments with open sights to telescopes armed with microm- 

 eters, and the result must have been little short of stunning to the 

 seventeenth century astronomers, tor it caused the hoary and gigantic 

 parallax of about 180" to shrink incontinently to 10", and thus ex- 

 panded their conception of the solar system to something like its true 

 dimensions. More than fifty years previously Kepler had argued from 

 his ideas of the celestial harmonies that the solar parallax could not 

 exceed 60", and a little later Horrocks had shown on more scientific 

 grounds that it was probably as small as 14"; but the final death blow 

 to the ancient values — ranging as high as 2' or 3' — came from these 

 observations of Mars by Flamsteed, Cassini, and Richer. 



Of course the results obtained in 1672 produced a keen desire on the 

 part of astronomers for further evidence respecting the true value of 

 the parallax, and as Mars comes into a favorable position for such 

 investigations only at intervals of about sixteen years, they had recourse 

 to observations of Mercuiy and Venus. In 1677 Halley observed the 

 diurnal parallax of Mercury, and also a transit of that planet across 



