THE SOLAR SYSTEM. 193 



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 upon the ancient instruments, and thus 

 the supposed measures of it were really nothing but measures of 

 their inaccuracy. 



The telescope was first pointed to the heavens by Galileo in 

 1609, but it needed a micrometer to convert it into an accurate 

 measuring instrument, and that did not come into being until 1639, 

 when it was invented by Wm. Gascoigne. After his death, in 

 1644, his original instrument passed to Richard Townley, who 

 attached it to a fourteen-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 published in the Philo- 

 sophical Transactions in 1667, and a little before that a similar 

 instrument had been invented by AuzouJ in France, but observa- 

 tories were fewer then than now, and so far as I know J. D. Cassini 

 was the only person beside Flamsteed who attempted to determine 

 the solar parallax from that opposition of Mars. Foreseeing the 

 importance of the opportunity, he had Richer despatched to 

 Cayenne some months previously, and when the opposition came 

 he effected two determinations of the parallax ; one being by the 

 diurnal method, from his own observations in Paris, and the other 

 by the meridian method from observations in France by himself, 

 Romer, and Picard, combined with those of Richer at Cayenne. 

 This was the transition from the ancient instruments with open 

 sights to telescopes armed with micrometers, and the result must 

 have been little short of stunning to the seventeenth century 

 astronomers, for it caused the hoary and gigantic parallax of about 

 t8o seconds to shrink incontinently to ten seconds, and thus 

 expanded their conception of the solar system to something hke 

 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 seconds ; and a little later Horrocks 

 had shown on more scientific grounds that it was probably as 

 small as 14 seconds, but the final death-blow to the ancient values 

 ranging as high as two or three minutes came from these observa- 

 tions of Mars by Flamsteed, Cassini, and Richer. 



