188 On the Catoptrical and Dioptrical 



the axes ; I say that two spectators placed, the one in E 

 and the other In I, so that the angles GCE and FCI may- 

 be equal, will see at the same time, the object placed at D; 

 the first through the object-glass AB, and the other by- 

 reflection from the second surface ACB of the same glass. 



30. " If the object-glass be very large, and its focal di- 

 stance great, the two spectators may be placed in the axis, 

 and they will both see the same object, though it be also si- 

 tuated in the axis, the one by refraction and the other by 

 reflection ; and this disposition of things will not in the 

 least hinder the effect. This property of large object-glasses 

 may be applied to useful purposes. 



3 1 . u Forttenelle, in the Hist. Acad, of Sc. for 1 700, after 

 describing the properties of the glass of Tschirnhausen, of 

 which we have spoken, says, that, K so many singularities in 

 this lens of Tschirnhausen announce great and happy disco- 

 veries in dioptrics. Although this science is yet in its in- 

 fancy, we shall be surprised at still making important dis- 

 coveries in it, so much are we of this age accustomed to the 

 rapid progress of the sciences/ 



32. u More than sixty years have elapsed since M. de 

 "Fontenelle made this prediction ; but I do not see that it is 

 yet accomplished. But this does not prove that the predic- 

 tion is not well founded ; for it frequently happens that dis- 

 coveries remain a long time unfruitful. For example, the 

 discovery of the reflecting telescope is almost as old as that 

 of the refracting one. This last was invented in loOQ, and 

 I find that the other began to be used as early as the year 

 16 16 ; so that between the two discoveries there is only an 

 interval of seven years. Father Zitcchi, an Italian Jesuit, 

 (in his Optica Philosophia, printed at Lyons in 1652, par. 1 . 

 cap. 14. § 5. p. 126.) says, that in 1616, reflecting on the 

 theory r of telescopes, then recently invented, it came into 

 his head to employ concave metallic mirrors, instead of 

 object lenses of glass, in order to produce by reflection the 

 same effects which result from refraction. Having found, 

 therefore, in a cabinet of curiosities a concave metallic mir- 

 ror, exactly worked by an able artist, he applied to it a con- 

 cave eye-glass, and with this telescope observed terrestrial 

 and celestial objects, and experience confirmed what theory- 

 had taught him. I do not think that a more antient date 

 of the reflecting telescope can be found. 



33. <l It appears, however, that this invention remained 

 unknown, or neglected, till Newton, a long time after, in 

 1672, constructed in London a reflecting telescope, which 

 presently became celebrated throughout Lurope. For al- 

 though;, 



