Instruments of the Antients, 1 H 



good effect, although its surface should not be greater than 

 that, of the lens in this experiment. 



21. ** -Every optician will easily conceive, that if, with a 

 single convex glass, he can see distant objects, by refrac- 

 tion, with as much clearness and distinctness as with a 

 telescope composed of several glasses, he will experience 

 the same effect, by reflection, from a single concave mirror. 

 I shall therefore state some experiments and observations 

 which prove incontestably, that this effect may be produced 

 with a single object-glass; which, consequently, will be a new 

 proof that a single concave mirror will do the same thing. 



22. « M. de Fontenelle, in his History of the Academy 

 of Sciences for the yeaf 1 700, tells us, that M. Tschirn- 

 hiiusen had described to the learned, the effects of a new 

 object-glass which he had made. Its focal distance was 

 32 feet : it was a double convex, a Rhenish foot (12 and 

 4-10ths English inches) in diameter. He used it without 

 an eye-glass, and altogether uncovered ; and he affirms that 

 objects were seen more clearly with his single lens than they 

 had ever been seen with telescopes^ and that they appeared 

 brighter than to the naked eye. Such a glass, then, will 

 answer its end without any tube, and the object will alwavs 

 be seen distinctly, notwithstanding the soiar rays, which 

 pass between it and the eye. The field, or the space which 

 may be viewed at once with such a lens, is of incredible ex- 

 tent. M. Tschirnhausen assures us that, without either 

 eye-glass or tube, he saw very distinctly, at noon-day, a 

 whole town, at the distance of a German mile and a half 

 (about six English miles). 



23. " After M. Tschirnhausen, the same thing was done 

 by M. Wolff, or Woljlus, with a large plano-convex, ttie 

 diameter of whose convexity was 30 Rhenish feet (31 Eng- 

 lish). This lens was two feet long and a foot and a half 

 broad (24 inches 8-10ths and 18 inches 6- lOths English 

 respectively). In his Elem. Diop, prob. 38. schol. 4. Wol- 



fius says, that in using this glass, which he left entirely open, 

 without either tube or eye-glass, he saw very distinctly, 

 with bbth eyes, some houses on the top of a hill, at the 

 distance of two German (or about eight English) miles. 

 He saw them erect, when he stood between the focus and 

 the lens, and inverted when he stood beyond the focus. 

 He observes, that his lens was by no means well polished** 



13. " Fa- 



* I cannot but observe that particular attention, in my opinion, is due 

 Jo this statement of IVolfius, whose candour and honesty appear to have 



O i been 



