7^ THREE QUESTIONS. 



confufion, and beyond that point I begin to difcern the inverted 

 image that hangs in the air, and is formed by the divergence 

 of the rays from their focal interfedions. 

 Apparent recefs Such are the fadls, but not the whole of the fa6ts. For I 



Jhe STmage *^^ "^^''^ ^"'^^' *^*^ ^^^^^ ^^® ^^^ receding of the ered image, 

 in a concave *it comes to a place where it is ftationary, and then inftead of 

 mirror. remaining at any fituation that may be called an infinite or 



more than infinite diftance, it comes forward again and conti* 



nues to advance till it is loft in confufion. 

 Difficulty ftated. Perhaps I may not have exprefied myfelf with all the clear- 



wV^dJi^c^c? "^^^ ^ ^°"^^ ^'^' ^"' ^ ^^'"^ ^ ^^^® ^^^" ^ difficulty and the 

 reafon why it puzzles me. The true fcience of nature muft 

 be adequate to the explanation of all phenomena, or at leaft 

 it muft not b6 contrary to any one of them. What is the fo» 

 lution of this ? 

 Optical mirror at 2. There is an optical mirror at Merlin's Exhibition in Han* 

 over-ftreet, in which you fee your face perpetually changing 

 from one diftortion to another, arifing from its greateft length 

 Singular diftor- varying through all its feveral diameters. So that the face be- 

 comes abfurdly long, crooked, broad, &c. If you mpvefar- 

 and revolution of ther off, the face has its natural afpecl, but is feen turning 

 ae image. round its center and fucceffively afiuming the ere6t and inverted 



pofitions, and every intermediate degree of obliquity, 

 A pojifhed but- While thefe phenomena, produced by an apparatus con- 

 ton of the fame cealed in a box, employed my occasional refledions, I hap- 

 pened to fee a polifhed metallic button in the ftiops, which 

 upon examination proves to be a mirror of the fame kind as 

 the larger one ufed at Merlin's. If it be confidered in thedi- 

 redion of one of its diameters, it is a concave fpeculum ; but 

 in the diredion of another diameter at right angles to the for- 

 mer, it is convex: and the radius of each curvature is about 

 one inch. It is the fame figure as a turner might produce in 

 the lathe by ufing a chizel with a circular edge of one inch 

 radius to fcoop out a portion of a revolving piece till its fraall- 

 eft diameter, or calliper, became equal to two inches. Its ef- 

 feds are, 

 particular de- When the eye is brought very near the poliflied furface, it 

 ^prlption of the appears elongated in the diredion of the diameter or chord, 

 bTtliebuuonr ^^ °^ *^^^ concavity, and fliortened in the diameter at right an- 

 gles to that diameter, and if the fpeculum be turned on its 

 Undulation. axis, the image is affeded with the ftrange undulation before 

 mentioned, and undergoes all the variations twice in one re- 



volutioa 



