464 SPECIAL PHYSIOLOGY. 



through space in one direction, as in railway travelling, objects appear 

 to move in the opposite direction, and near objects appear to pass us 

 relatively faster than more distant ones, for the angular motion of 

 objects is greater, the closer their proximity to the eye. This assists 

 us in judging of the rapidity and uniformity of our own movement, as 

 well as of the distance of objects. If we look intently at certain ob- 

 jects in motion, after a time they seem to be at rest, and ourselves 

 appear to be in motion ; thus, on watching a running stream from a 

 bridge over it, the stream soon appears to be still, whilst we seem to 

 be moving with the bridge, in the opposite direction. 



Such being the modes in which sight is employed for the purpose of 

 informing us of the presence, size, shape, position, and movements of 

 external objects, there yet remain many phenomena of vision which 

 require to be considered. 



For the stimulus of light to produce an impression upon the retina, 

 it is essential that the portion of its surface acted upon by the luminous 

 rays, be of a certain size, that these rays be of a certain strength, and 

 that the retina be exposed to their action for a certain period of time. 

 Images of luminous objects of a certain magnitude, might be regarded 

 as mosaic patterns, composed of an infinite number of minute luminous 

 points ; theoretically, these are infinitely smaller than the ends of the 

 retinal rods. It has been calculated that a body, the image of which 

 occupies only $?9jjth of an inch of the retina, is visible to the eye, 

 which is considerably less than the diameter of a single retinal rod or 

 cone. It is said, however, that two impressions are distinctly or sep- 

 arately perceived, only when a certain distance intervenes between 

 them ; this is said to correspond with the diameter of the cones. 

 White bodies on a black surface, or black bodies on a white surface, 

 which measure as little as 4Q(jth of an inch in diameter, can be seen 

 by the naked eye ; bodies still smaller can be perceived, if the atten- 

 tion be fixed, and the light powerful, though their outline becomes 

 indistinct; but if less than gj^th of an inch in diameter, they are no 

 longer visible, although they can still be seen when arranged in rows. 

 Lines, such as opaque threads, are visible, ev,en when only 433 ^th of 

 an inch in thickness. Bright bodies of exceedingly small size are 

 visible, which, were they less brilliant, would be invisible. If the light 

 be not of a certain intensity, the retina is not aroused to vision ; the 

 appearance of darkness in this case, is dependent, not on a total ab- 

 sence of light, for it is still present, but on its want of intensity. 

 When an opaque body passes before the eyes with great velocity, the 

 period of time during which it is visible, is so brief, that the retina 

 receives no impression of the object; hence cannon-balls are invisible 

 when they pass before us, but not always when they approach or re- 

 cede from us. Luminous bodies, on the other hand, however rapid 

 their course, and whatever their direction, are always visible. 



Luminous rays from a small object, falling on the retina, not only 

 excite impressions in that portion of the retina which receives them, 

 but also in the surrounding surface for a certain distance. The stim- 

 ulation of the retina by the luminous rays, is, it would seem, diffused 



