THE EYE. 



same time by the retina from other objects present before the eye. 

 The proof of this is, that the same objects which are visible at 

 one time are not visible at another, though equally before the eye, 

 and transmitting equal quantities of light of the same intensity 

 to the retina. Thus, the stars are present in the heavens bv day 

 as well as by night, and transmit the same quantity of light to 

 the retina, yet they are not visible in the presence of the sun, 

 because the light proceeding from that luminary, directly and 

 indirectly reflected and refracted by the air and innumerable other 

 objects, is so much greater in quantity and intensity as to over- 

 power the inferior and much less intense light of the stars. This 

 case is altogether analogous to that of the ear, which when under 

 the impression of loud and intense sounds, is incapable of per- 

 ceiving sounds of less intensity, which nevertheless affect the 

 organ in the same manner as they do when, in the absence of 

 louder sounds, they are distinctly heard. 



Even when an object is perceived, the intensity of the percep- 

 tion is relative, and determined by other perceptions produced at 

 the same time. Thus, the moon seen at night is incomparably 

 more splendid than the same moon seen by day or in the twilight, 

 although in each case the moon transmits precisely the same 

 quantity of light, of precisely the same intensity, to the eye ; but 

 in the one case the eye is overpowered by the superior splendour 

 of the light of day, which dims the comparatively less intense 

 light proceeding from the moon. 



48. 4. THE IMAGE MUST CONTINUE A SUFFICIENT TIME UPON 

 THE EETINA TO ENABLE THAT MEMBHANE TO PEODUCE A PEE- 

 CEPTION OF IT. 



The velocity with which light is propagated through space is at 

 the rate of about 200000 miles per second. Its transmission, 

 therefore, from all objects at ordinary distances to the eye may be 

 considered as instantaneous. The moment, therefore, any object 

 is placed before the eye an image of it is formed on the retina, 

 and this image continues there until the object is removed. Now 

 it is easy to show experimentally that an object may be placed 

 before the eye for a certain definite interval of time, and that a 

 picture may be painted upon the retina during that interval 

 without producing any perception or any consciousness of the pre- 

 sence of the object. 



To illustrate this, let a circular disk A B c D, fig. 5, about twenty 

 inches in diameter, be formed in card or tin, and let a circle 

 A' B' c' D' be described upon it, about two inches less in radius 

 than the disk, so as to leave between the circle and the disk a 

 zone about two inches wide. Let the entire zone be blackened, 

 except the space A M M' A', forming about the one-twentieth of it. 

 68 



