22 HISTORY OF 



ture 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 through the small aperture as come 

 in opposite directions. Thus the little aperture 

 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 objects 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 sensa- 

 tions may seem to tell us to the contrary. For, 

 to convince us of this, we have only to compare 

 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 

 we close the other eye, we shall find that part of 

 the room visible, and the finger seeming to cover 

 a part 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 



