ANSWERS TO OPTICAL QUESTIONS. 2i37 



or the phenomenon of two images. In proof of this, if he will 



take two pens or pencils or other pointed objedls, and, fliutting 



one eye, endeavour to apply tlieir points end-wife to each other but on the 



by moving them acrofs the line of fight, he will find how ^'y^^^''^^^^^^^^^* 



much lefs precifely one eye can give a judgment on diftances, perfpeftive. 



than both eyes. And our judgment of diftant objects is efta- 



bliflied not at all upon the nature of the pencils, but on the 



perfpedive arrangement or angular magnitude of the objects 



together with their gradual obfcuration (from the interpofed 



mafs of air) called aerial perfpedtive. Hence we find that the 



feveral parts of a well drawn tketch of mere lines, and flill 



more of an excellent painting, give all the notions of diftance, 



though there cannot be any difference in the divergence of the 



pencils of light to correfpond with that effeft. The confciouf- 



nefs that a body is approaching or retiring arifes little if at all 



from any change in the pencils ; in near objedts that notion is 



rather gained from the required change in the optical axes, 



and in thefe and all diftant objects it is almoft entirely produced 



by the angular magnitude becoming larger or fmaller, while 



we contemplate the appearances. 



Thefe truths will explain the image in the concave mirror : The image In 



For 1. When the face is near the mirror, the image is feen by ^ concave niirroF 



r , • I . . , , . :; ^^^'"s to retire 



a great convergence ot the optical axes, which becomes lefs and advanct 



and lefs as the obferver retires, at the fame time that the image ^^°^ ^^^^'^ 

 itfelf fubtends a lefs and lefs angle: — It therefore, for both 

 reafons, appears to retire. 2. At a greater djftance the image 

 feems ftationary while the obferver retires, becaufe the angular 

 enlargement of the image is nearly compenfated by its dimi- 

 nution from increafed diftance; and 3. At a ftill greater dif- 

 tance the angular enlargement increafes fo faft that the image 

 feems rapidly to come forward until it is loft in confufion.— 

 If both eyes be kept open, the obferver has a contiderably* 

 accurate notion of the real diftance of the frame, which, to- 

 gether with the confufion of two different images prefented to 

 the eyes, may lead him to have the fentiment of a mere angu- 

 lar enlargement; but if one eye be covered, the notions of re- 

 treat, ftationary pofilion, and rapid advance of the image will 

 infallibly be adopted. 



2. With regard to the concavo-convex mirror, none of the 



pencils of light are brought truly to a focus. If we conceive 



the furface of the mirror to be divided into zones by lines 



4 drawn 



