Chap. 7.] How we judge of Diftance. 243 



the fame real magnitude ; hence it learns by degrees 

 to combine together the angle under which an object 

 is feen and the diftance, and, according to its future 

 employments in life, thefe ideas w.ill be combined to- 

 gether with greater or lefs accuracy. The judgment 

 of one perfon, accuftomed to diftant objects, will be 

 very correct, while a perfon employed in the nicer 

 works of art will be continually deceived in looking at 

 the fame diftant objects, and the contrary. 



An object will affect us differently, I have faid, ac- 

 " cording to the angle under which it is feen, and its dif- 

 tance. In figure 40 let AB be an object viewed di- 

 rectly by the eye Q_R. From each extremity draw 

 the lines AN and B M, interfering each other in the 

 cryftajline humour at I. Then draw the line I K in. 

 the direction in which the eye is fuppofed to look at 

 the object. The angle A IB is then the optical or vi- 

 fual angle, and the line I K is called the optical axis, 

 becaufe it is the axis of the lens or cryftalline humour 

 continued to the obje6t. 



The apparent magnitude of objecls then, depending 

 thus on the angle under which they are feen, will evi- 

 dently vary according to their distances. Thus dif- 

 ferent objects, as A B, C D, E F, the real magnitudes 

 of which are very unequal, may be fituated at fuch 

 diflances from the eye as to have their apparent mag- 

 nitudes all equal ; for if they are fituaj/bd at fuch dif- 

 tances that the rays AN, B M, mail touch the extre- 

 mities of each, they will then appear all under the fame 

 optical angle, and the diameter M N of each image on 

 the retina will confequently be equal. 



In the fame manner objedls of equal magnitude, 



fituated at unequal diftances will appear unequal. For 



let A B and G H, two objects of equal fize, be placed 



before the eye at different diftances I K and I S ; draw 



R 2 the 



