1S6 0/7 the Catoptrical and Dioptrical 



18. " Thus the learned knew, before M. Tschirnhamcn 

 taught them, that, with a single object-glass, distant ob- 

 jects might be viewed, as with a telescope composed of se- 

 veral glasses. I believe, however, that Tschirnhansen, when 

 he published the properties of his large object-glass, had no 

 knowledge of what father Scheiner had written on the sub- 

 ject, or of what had been executed at Milan by Septala, 

 who discovered the effect of his lenses, without having seen 

 Schemer. There may even be others who may have ob- 

 served this property of large convex, glasses, without having 

 had any knowledge of what had been written or observed by 

 the authors I have cited. 



19. iC I never could procure such large object-glasses as I 

 wished, in order to repeat the experiments of these authors, 

 and to make many more, which I have thought of. I have, 

 however, made some experiments with the largest lenses, 

 which I had it in my power to make or procure. I shall 

 here give the results : 



20. Ci (I.) With an object-glass, whose focal distance 

 was about three feet, and diameter about six inches, by pla- 

 cing my eye farther from the class than the focus, I saw di- 

 stant objects clearly and distinctly, considerably enlarged 

 and inverted. This glass was a double-convex. 



21. " (II.) I observed the same thing, with still greater 

 clearness and distinctness, with a plano-convex object-glass, 

 a segment of a sphere about six feet in diameter, its own di- 

 ameter being about seven inches. 



22. " (III.) The clearness and distinctness, with which 

 I saw the same distant objects, were much increased when I 

 used a double-convex- of about six inches and a half in 

 breadth, and twenty-two feet focal distance. With this 

 object-glass, I had a very accurate and distinct view of ob- 

 jects, which I could by no means have distinguished so well 

 with a good telescope. 



ei Remarks on the experiments which may he made with 

 large object-glasses, and with others. 



23. " (I.) In all my experiments with these three glasses, 

 I always saw the objects with the most clearness and di- 

 stinctness when they were inverted, by placing the eye be- 

 yond the focus. 



24. " (II.) The two first glasses did not produce a good 

 effect, when, the eye being between the focus and the glass, 

 the objects were seen erect; and the same is true of all other 

 object-glasses of which the focal distance is not very con- 

 siderable. 



25. " (III.) 



