92 THE HISTORY OF SCIENCE 



himself for a few days to the problems of mensura- 

 tion involved, and in 1615 published his treatise 

 (Stereometria Doliorum) on the cubical contents of 

 casks (or wine-jars), a source of inspiration to all 

 later writers on the accurate determination of the 

 volume of solids. He helped other scientists and was 

 himself richly helped. As early as 1610 there had 

 been presented to him a means of precision of the 

 first importance to the progress of astronomy, 

 namely, a Galilean telescope. 



The early history of telescopes shows that the 

 effect of combining two lenses was understood by 

 scientists long before any particular use was made of 

 this knowledge ; and that those who are accredited 

 with introducing perspective glasses to the public 

 hit by accident upon the invention. Priority was 

 claimed by two firms of spectacle-makers in Middel- 

 burg, Holland, namely, Zacharias, miscalled Jansen, 

 and Lippershey. Galileo heard of the contrivance 

 in July, 1609, and soon furnished so powerful an 

 instrument of discovery that things seen through 

 it appeared more than thirty times nearer and al- 

 most a thousand times larger than when seen by the 

 naked eye. He was able to make out the mountains 

 in the moon, the satellites of Jupiter in rotation, 

 the spots on the revolving sun ; but his telescope 

 afforded only an imperfect view of Saturn. Of 

 course these facts, published in 1610 (Sidereus Nun- 

 cms), strengthened his advocacy of the Copernican 

 system. Galileo laughingly wrote Kepler that the 

 professors of philosophy were afraid to look through 

 his telescope lest they should fall into heresy. The 

 German astronomer, who had years before written 



