$$4 On the Catoplrical and Dioptrical 



87. " Among the great number of mirrors considered as 

 plane, there are some so sufficiently and regularly concave, 

 as to produce the effect of concaves, when the mirror, the 

 object, ,and the spectator happen to be properly situated. 

 Father Zahn, in his Oculus Artificialis Teledioptricus Fun- 

 daw. 3. Sijntag. 3. cap. 4. Prob. 6. mentions a singular oc- 

 currence which happened to a canon of Erfurt, and which 

 proves my assertion. This canon walking one day in his 

 apartment, happening to look at a mirror hanging on the 

 wall, saw a crucifix as large as life, which seemed to be the 

 same with that on the altar in his church. On changing 

 his situation it disappeared, but when he returned to the 

 same spot he saw it again. Looking around he could see 

 nothing to which he could ascribe so large a figure; but at 

 last he perceived, in an elevated situation, a small image of 

 a crucifix; on removing which, and returning to his first 

 situation, he no longer saw the image in the looking-glass. 

 Not being able to account for so s nail an image appearing 

 so large in a mirror, which he had always considered as a 

 plane one, he mentioned the phaenomenon to father Zahn, 

 who very properly told him, that his pretended plane mirror 

 was really concave; that the diameter of its concavity being 

 great, it did not sensibly enlarge the faces of those who 

 stood near it, but that, at a considerable distance, they must 

 of course be magnified. He added, that " Plura similia, 8cc. 

 Many such things occur, which appear altogether astonish-' 

 ing to those who are ignorant of their causes.' * 



88. u I never yet met with a mirror, which, being accu- 

 rately examined, turned out to be perfectly plane; but 1 have 

 met with more than one supposed plane mirror, which were 

 not only concave, but of a tolerably regular concavity; sq 

 that when, from a considerable distance, [ viewed remote 

 objects in them, I saw those objects sensibly enlarged with- 

 out being disfigured. 



89- " It is evident from Zahn y s observations and my own, 

 that among mirrors given to us as plane ones, we may hap- 

 pen to find some which are concave, whose focal di- 

 sance is 30, 40, 50, &c. feet, and whose curvature is at the 

 same time regular. I will add, that there is a greater chance 

 of our meeting with mirron of a regular concavity or con- 

 vexity than with those which are perfectly, plane. For ar- 

 tists, who have been long employed on large object-glasses 

 for telescopes, know that it is much easier to make glasses 

 perfectly spherical than perfectly plane. If necessary, it 

 would even be easy to demonstrate rigorously, that let mir- 

 rors intended to be plane, be worked how they may, they 



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