Instruments of the Anticnts* -is; 



25. u (III.) In using the third object-glass, although I 

 saw the objects clearly, by standing between the focus and 

 the lens, it was not without some contusion. Yet persons 

 whose eyes unite the rays from distant objects beyond the 

 retina, by standing between the focus and the glass, will 

 see objects erect with much clearness and distinctness ; in- 

 deed as well as with the best telescope, and even better. 



26. " (IV.) Objects seen through great lenses appear 

 much more bright than they do to the naked eye, and as if 

 they had a stronger light thrown on them. This effect is 

 so sensible, that other objects seen with the naked eye, be- 

 side those looked at through these glasses, appear obscure, 

 although equally illuminated. The direct contrary takes 

 place, when objects are viewed through telescopes ; for they 

 always appear less illuminated than when seen by the naked 

 eye. Hence it seems very probable, that these convex glasses 

 and concave mirrors might be used with success, to view by 

 night, objects which cannot be seen only because of ihe de- 

 fect of light; for, happening in a very dark night to try the 

 glass first mentioned, I saw distinctly objects which, with- 

 out it, I could not see at all. 



27. " (V.) Through a single great object-glass, several 

 persons may view different objects at the same instant. I 

 have frequently observed that, in order to distinct vision, it 

 is not necessary that the eye be in the axis ; for, though it 

 be some degrees distant from the axis, the effect is not sen- 

 sibly diminished. 



28. i( But, what is yet more extraordinary in these large 

 object-glasses is, that two spectators, without disturbing 

 each other, may, by means of the same single glass, view 

 the same object at the same time. In order to this, one of 

 the two spectators must be so placed that the lens may be 

 between him and the object, his eve not being in the axis, 

 but in a line which, passing through the point of concourse 

 of the axes, and through the object, makes a small angle 

 with the axis. Then, if a second spectator stand between 

 the object-glass and the object, so that a line passing through 

 his eye and the point of concourse of the axes may make 

 with the axis an angle equal to that which a line passing 

 through the eye of the first spectator and the same point of 

 concourse, and both spectators be on the same side of the 

 axis ; I say then, that both of them will see the same object 

 at the same time, the first by refraction, and the secondf by 

 reflection. 



29. "Let AB (fig. 4. Plate ITT.) be a large object-glass, 

 of which FG is the axis, and C the point of concourse of 



04 the 



