GALILEO'S INVENTION OF THE TELESCOPE. 175 



true that the concave glass diminishes, and that the con- 

 vex one increases them ; but both show them very in- 

 distinctly, and hence one glass is not sufficient to produce 

 the effect. Passing on to two glasses, and knowing that 

 the glass of parallel superficies has no effect at all, I con- 

 cluded that the desired result could not possibly follow by 

 adding this one to the other two. I therefore restricted 

 my experiments to combinations of the other two glasses ; 

 and I saw how this brought me to the result I desired. 

 Such was the progress of my discovery, in which you see 

 of how much avail was the knowledge of the truth of the 

 conclusion. But Signor Sarsi* or others, believe that the 

 certainty of the result affords great help in producing it 

 and carrying it into effect. Let them read history, and 

 they will find that Archites made a dove that could fly, 

 and that Archimedes made a mirror that burned at great 

 distances, and many other admirable machines. Now, by 

 reasoning on these things, they will be able with very 

 little trouble, and with very great honour and advantage, 

 to discover their construction ; but even if they do not 

 succeed they will derive the benefit of being able to certify, 

 for their own satisfaction, that that ease of fabrication 

 which they had promised themselves from the pre-know- 

 ledge of the true result is very much less than what they 

 had imagined.' 1 



Those who evolve the lesser truths which enable great 

 minds to co-ordinate those truths and discover general 

 principles, are usually, though not necessarily, minds of 

 lesser capability. Some of them would have been equally 

 great, and would have discovered the same general truths, 

 if the conditions had been equally ripe for their dis- 

 covery. Great ideas usually germinate in greater or less 



1 Conferences. Special Loan Collection, London, 1876. Address by 

 Professor De Eccher, pp. 105, 106. 



