

Instruments of ihe Antienls. 1 8 1 



clothed it, place the principal fact beyond the bounds of 

 probability, or demonstrate its impossibility. 



10. " It is evident that the possibility of the principal 

 fact, and of the foundation of this story, consists in the 

 possibility of a mirror, which could alone enable us to see 

 objects at as great a distance and with as much clearness 

 and distinctness, as a good telescope. The proofs of this 

 possibility depend on several delicate experiments in catop- 

 trics and dioptrics, of which many persons conversant with 

 those sciences are still ignorant. 



11. " We shall have little trouble in discovering the 

 source of the absurdities, which have been mixed with the 

 history of this fact, if we prove that the execution of such 

 a mirror is possible, and that it was also possible in the 

 days of Ptolemy. This will not be difficult ; and I propose 

 to do it in the present memoir, which I shall divide into 

 two parts. In the first, 1 shall prove, by the strictest evi- 

 dence, the possibility of this mirror ; in the second, I shall 

 deliver some proofs that it really existed. 



12. (i Part First. Proof of the possibility of the fact. 

 " Let a concave mirror be constructed, of a great size, 



and forming a portion of a large sphere : I say, that in such 

 a mirror we may see objects as distant, and with as much 

 clearness and distinctness, as with a good telescope. 



13. " In order to this, the observer must stand between 

 the object and the mirror, but so as not to intercept any of 

 the rays. The mirror then should be so placed, that the in- 

 cident ravs coming from the object, and the rays reflected 

 by the mirror to the eye, may make a small angle with the 

 axis. This angle ought not to be greater than is necessary, 

 in order that all the rays which come from the object to the 

 mirror may pass by the observer. Things being thus ar- 

 ranged, I establish the following facts : 



14. u First, If the eye be near the focus, or the place 

 where the image of the object is formed, and between the 

 focus and the mirror, the object will be seen much mag- 

 nified and in the natural situation. But here we must re- 

 mark, that if the observer be a myope, or short-sighted, he 

 will sec nothing but a confused appearance. It' his sight 

 be good, and he be neither a myope, nor a presbyte, he will 

 distinguish the objects well, and will sec them much mag-, 

 nified, but with some confusion and obscurity, unless the 

 mirror be a portion of a very large sphere. Lastly, if the 

 observer be a presbyte, or long-sighted, he should stand at 

 the distance at which he sees the objects direct and much 



Vol. 19. No. 74. July 1S04. O niagniiied. 



