244 Wh' an extended Plain appears elevated. [B ook III. 

 the lines G P and H O, eroding each other in I ; then 

 O P, the image formed by the object G H on the re- 

 tina, is evidently of a greater diameter than the image 

 M N, which reprefents the object A B ; in other words, 

 the object G H will appear as large a$ an object of the 

 diameter T V fituated at the fame place as the object 

 AB. 



Hence it follows, that objects fituated at different 

 diftances, whole apparent magnitudes are equal, are to 

 each other as their diftances from the eye ; and by the 

 fame rule, equal objects fituated directly before the 

 eye, have their apparent magnitudes in a reciprocal 

 proportion to their diftances. 



This laft propofition muft, however, be received with 

 lome allowance j for it is only applicable to very dif- 

 tant objects, and to thole where the fenfe is not cor~ 

 ' reeled by the judgment. For if the objects are near, 

 we do not judge of their magnitude according to the 

 vifual angle. Thus, if a man of fix feet high is feen 

 at the diftance of fix feet under t'he very fame angle as 

 a dwarf of only two feet high at the diftance of two 

 feet, ftill the dwarf will not appear as large as the man, 

 becaufe the fenfe is corrected by the judgment. 



In moft cafes, however, where the diftance is confi- 

 dcrable the rule will be found accurate ; and as it has 

 its foundation in nature, moft of the phenomena of vi- 

 fion will be explained by having racourfe to the prin- 

 ciples laid down in this chapter. If the eye is placed 

 above a horizontal plain, the different parts of this 

 plain will appear elevated in proportion to their dif- 

 tance, till at length they will appear upon a level with 

 it. For in proportion as the different parts are more 

 diftant, the rays which proceed from them form angles 

 with the optical axis I K more and more acute, and at 

 length become almoft parallel. This is the reafon 



why, 



