GALILEO. 107 



which the temperature is indicated by the expansion of the liquid her- 

 metically sealed in the glass vessel. At a later period (1670) mercury 

 was substituted for spirits of wine. 



During this period of his professorship at Padua, Galileo became 

 convinced of the truth of the astronomical doctrines of Copernicus. 

 For some time after he had himself become a convert he was obliged 

 to continue teaching the Ptolemaic system, out of compliance with 

 popular prejudices. In 1594 a remarkable witness against the Aris- 

 totelean doctrines appeared in the form of another new star in the con- 

 stellation Ophiuchus, as if to protest against the dogma of the un- 

 changeability of the heavens. A similar phenomenon had been observed 

 thirty years before, when Galileo was a mere child. On this occasion 

 a crowd of auditors filled the philosopher's lecture-room to learn the 

 results of his observations. He showed from the absence of parallax 

 that the new star could not be, as was generally alleged, merely a me- 

 teor at no great distance from the earth, but that it was really situated 

 among the remotest heavenly bodies. This reasoning was unanswerable, 

 as the reader may perceive who will be at the trouble to understand 

 what is meant by parallax. This may be explained by a familiar, ex- 

 perience. He has probably observed when travelling on a railway the 

 rapid change produced in the relative positions of the objects of the 

 landscape by the change in his point of view. When a wide extent of 

 country is thus rapidly traversed, the objects almost appear to be re- 

 volving round a centre which is being continuously displaced as the 

 train proceeds. He has also doubtless observed that the distant ob- 

 jects do not exhibit changes in their relative positions to anything like 

 the same degree. A distant spire or mountain will, for instance, retain 

 nearly the same apparent position against the sky while the traveller 

 changes his point of view by a considerable distance. Hence it is 

 obvious that if by changing our point of view we produce no percep- 

 tible change in the relative positions of a group of objects, it can only 

 be because these objects are at a distance very great as compared with 

 the change of our position. Therefore, as the new star in Ophiuchus 

 had precisely the same position among the other stars from whatever 

 part of the earth it was viewed, it must obviously have been at an 

 immense distance from us. 



Galileo's popularity at Padua became so great that his lecture-room 

 was thronged with auditors, and a place capable of holding a thousand 

 persons was at times too small for his audience. After he had held 

 the professorship at Padua for nearly eighteen years, overtures were 

 made which induced him to return to his former position at Pisa, on 

 terms which would leave him ample leisure for the prosecution of his 

 researches and the composition of his works. The period immediately 

 preceding Galileo's retirement from Padua was marked by the great 

 invention with which his name will ever be associated the Telescope, 

 although the honour of priority has been claimed for others. Passages 



