246 Moon fe ems io move inftead of Clouds. [Book III. 



trary direction. Thus we attribute to the fun and the 

 other heavenly bodies, a diurnal motion, which only 

 affects the earth which we inhabit. 

 . If two or three objects at a confiderable diftance, 

 and on which the eye of. the fpectator is fixed, move 

 with equal velocity paft a third object which is at reft, 

 the moving objects will appear to be actually at reft, 

 and that which is really ftationary vnll appear in mo- 

 tion. Thus the clouds which pafs over the face of 

 the moon appear at reft, while the moon itfelf appears 

 to proceed rapidly along in an oppofite diiection. 

 This happens, bccaule the eye which is fixed upon the 

 clouds follows their motion mechanically, and there- 

 fore the moon appears to move and not the clouds, as 

 in the boat we do not pe/ceive its motion, but con- 

 ceive the banks are retiring behind us. 



If the center of the pupil, that is, the optic axis is 

 directed along the furface of any flender object in a 

 perfectly right line, this line will appear only a point, 

 becaufe, in fact, the extremities only are vifible. 



An extended and cliftant arch, viewed by an eye 

 which is exactly in the fame line, will appear a.s a plane 

 furface, becaufe all the pares appearing equally diftant, 

 the curvature will not be perceived. 



If a circle is viewed obliquely it will appear an oval, 

 becaufe the diameter which is perpendicular to the eye 

 is fhortened ; in other words, the rays which proceed 

 from the extremities form an angle fo much the more 

 acute as the obliquity is greater ; on the contrary, the 

 diameter which is parallel to the eye is apparently ex- 

 tended. 



To fee any object it has been cbferved (Fig. 39.) 

 that an imprelnon muft be made upon the retina ; but 

 to the eye, as to the other fenfes, it happens, that an 

 imprcflion does not always produce a lentatio.n. There 



is 



