ANIMALS. 3.51 



dark room, so that no light can come in, but through it, all the 

 objects wdthout will be painted on the wall behind, but in an in- 

 verted position, their heads downwards. For as all the rays 

 which pass from the different parts of the object \vithout, can- 

 not enter the hole in the same extent which they had in leaving 

 the object ; since, if so, they would require the aperture to be 

 as large as the object ; and, as each part and every point of the 

 object sends forth the image of itself on every side, and the rays 

 which form these images pass from all points of the object as 

 from so many centres, so such only can pass tlu-ough the small 

 aperture as come in opposite directions. Thus the little aper- 

 ture becomes a centre for the entire object ; through which the 

 rays from the upper parts, as well as from the lower parts of it, 

 pass in converging directions -, and consequently, they must 

 cross each other, in the central point, and thus paint the ob^k.cts 

 behind, upon the wall, in an inverted position. 



It is in like manner, easy to conceive, that we see all objects 

 double, whatever our present sensations may seem to tell us to 

 the contrary. For to convince us of this, we have only to com- 

 pare the situation of any one object on shutting one eye, and 

 then compare the same situation by shutting the other. If, for 

 instance, we hold up a finger and shut the right eye, we shall 

 find it hide a certain part of the room ; if again re-shutting the 

 other eye, we shall find that part of the room visible, and the 

 finger seeming to cover a ])art of the room that had been visible 

 before. If we open both eyes, however, the part covered will 

 appear to lie between the two extremes. But the truth is, 

 we see the object our finger had covered, one image of it to the 

 right, and the other to the left ; but, from habit, suppose that we 

 see but one image placed between both ; our sense of feeling 

 having corrected the errors of sight. And thus, also, if instead of 

 two eyes, we had two hundred, we should, at first, fancy the ob- 

 jects increased in proportion, until one sense had corrected another. 

 " The ha^dng two eyes might thus be said to be rather an in- 

 convenience than a benefit ; since one eye would answer the 

 purposes of sight as well, and be less liable to illusion. But it 

 is otherwise ; two eyes greatly contribute, if not to distinct, at 

 least to extensive vision.' When an object is placed at a mo 



1 l.eniiiirdi) da Viiu-i. 



