POWER OF SHORT SPACES. 



33 



retina of the eye as circles, inasmuch as the perforations remain 

 intact, and their position unaltered, they are not perceived as 

 such, because the surface at the bottom of the eye on which 

 ihe inner half of each falls is illuminated. Hence they appear 

 as semicircles (see fig. 5). 



Fig. 4. 



Fig. 5. 



:0: 



From which it is manifest, that however well defined an 

 object may be, and however assured we may feel that its image 

 is actually portrayed on the bottom of the eye, it is not recoj^- 

 nised unless the contiguous surfaces are oppositely affected with 

 respect to light and shade. 



The second experiment is the converse of tlie last, and goes 

 to prove that an image is visible onhj token the retina of the eye 

 and the object are unequally illuminated. Let that portion of a 

 common sewing-needle which contains the eye be mounted on 

 a slip of glass as if for the microscope ; and let the paper 

 with which it is covered, have a very small circular aperture 

 through which to examine it, thus (fig. 6) : 



Fiff. 6. 



On holding the object close to the naked eye, it is found 

 to be altogether invisible. Nothing is seen but vacant space. 

 It is matter of certainty, however, that the front rays are in- 

 tercepted, and that a sliadow of the needle is therefore really 

 formed, but before reaching its destination, lateral rays stream 

 into the eye in all directions, which neutralise the shadow, 

 and so nothing is seen (fig. 7). 



VOL. IV. D 



