THREE QUESTIONS. . 7| 



XVI. 



Quejiions, 1 . rejpe6ting the Place of the ere6i Image behind a 



Concave Speculum; 2. and of the Image formed by a ConcavO" 



convex Mirror which is not Left-handed , and has the Property 



of revolving on its Axis along with the Mirror, but twice as 



faji; and 3. the Figure of the Sky. By a Correfpondent, 



To Mr. NICHOLSON, 

 S I R, 



1 HAVE a general knowledge of the deiightful fcicnce of Queftlons pro- 

 optics, but have no pretenlions to an intimate acquaintance P"'**** 

 with its depths. I have therefore prefumed to requeft you will 

 either give your own fentiments on the following difficulties, • 

 or lay them before your readers, fome of whom I hope will give 

 a popular explanation of them. 



I . We are taught by all the writers on vifion that the diftance General ftate. 

 of any objefl is afcertained from the divergence of the rays means\y which 

 from any affumed point on its furface, and that as the fenfe or we infer the 

 judgment refers to the point of divergence, we may have ^ 5°^ ^°"*^^'fi 

 perception of an optical image by reference to that region or ble obje^s. 

 part of fpace from which the rays in their latl courfe are di- 

 re6led, whether they have in abfoliite fact proceeded from this 

 laft point or not. Thus it is that we fee an image behind a Looking ghft. 

 looking glafs though the rays have never actually proceeded 

 from that image ; but only move in the fame directions as if 

 they had done fo. If the mirror be convex, the image will 

 be nearer to its furface, becaufe the rays will be more fcat- 

 tered and divergent, and fee m to have come from a neater 

 point. And parallel rays are faid to come from an objed in- 

 finitely diftant. 



Now Sir, my query refpefting the concave mirror is this:— Phenomena of 



When I hold my face dole to the furface, the image and ob- ^^^ ^"^^S" in a 

 ._ , ,, T£T ji ^ • , concave mirror, 



jecl touch each other. It 1 recede, the erect image recedes 



as the rays become iefs and lefs divergent. Every one knows 

 that when I am at the diftance of half the radius, the rays are 

 returned parallel, andconfequently the image is then infinitely 

 diftant. When I move farther off, the rays become conver- 

 gent, and therefore in a certain fenfe may denote an image far- 

 ther off than at an infinite diftance ; but I have no with to dif- 

 cufs this point. When I am at th« center of the fphere all is 

 5 con- 



