186 DISSERTATION SECOND. [i-art i. 



however, as is usual in such cases, has not been quite sa- 

 tisfactory ; and all that is known with certainty is, that the 

 honour belongs to the town of Middleburgh in Zealand, 

 and (hat the date is between the last ten years of the six- 

 teenth century, and the first ten of the seventeenth. Two 

 different workmen belonging to that town, Zachariah Jans, 

 and John Lapprey, have testimonies in their favour, be- 

 tween which it is difficult to decide ; the former goes back 

 to 1590, the latter comes down to about 1610. It is not 

 of much consequence to settle the priority in a matter 

 which is purely accidental ; yet one would not wish to for- 

 get or mistake the names of men whom even chance had 

 rendered so great benefactors to science. What we know 

 with certainty is, that the account of the effect produced 

 by this new combination of glasses being carried to Galileo 

 in 1610, led that great philosopher to the construction of 

 the telescope, and to the interesting discoveries already 

 enumerated. By what principle he was guided to the 

 combination, which consists of one convex and one con- 

 cave lens, he has not explained, and we cannot now exactly 

 ascertain. He had no doubt observed, that a convex lens, 

 such as was common in spectacles, formed images of ob- 

 jects, which were distinctly seen when thrown on a wall or 

 on a screen. He might observe also, that if the image, 

 instead of falling on the screen, were made to fall on the 

 eye, the vision was confused and indistinct. In the trials 

 to remedy this indistinctness, by means of another glass, 

 it would be found that a concave lens succeeded when 

 placed before the eye, the eye itself being also a little more 

 advanced than the screen had been. 



This instrument, though very imperfect, compared with 

 those which have been since constructed, gave so much 

 satisfaction, that it remained long without any material 

 improvement. Descartes, whose treatise on Opticks was 



