SEC. 2. THE FACTS OF ACCOMMODATION. 



708. When an object emitting or reflecting light, a lens, 

 and a screen to receive the image of the object, are so arranged 

 in reference to each other, that the image upon the screen is 

 sharp and distinct, the rays of light proceeding from each 

 luminous point of the object are brought into focus on the screen 

 in a point of the image corresponding to the point of the object. 

 If the object be then removed farther away from the lens, the 

 rays proceeding in a pencil from each luminous point will be 

 brought to a focus at a point in front of the screen, and, subse- 

 quently diverging, will fall upon the screen as a circular patch 

 composed of a series of circles, the so-called diffusion circles, 

 arranged concentrically round the principal ray of the pencil. If 

 the object be removed, not farther, but nearer the lens, the pencil 

 of rays will meet the screen before they have been brought to 

 focus in a point, and consequently will in this case also give rise 

 to diffusion circles. When an object is placed before the eye, so 

 that the image falls into exact focus on the retina, and the pencils 

 of rays proceeding from each luminous point of the object are 

 brought into focus in points on the retina, the sensation called 

 forth is that of a distinct image. When on the contrary the object 

 is too far away, so that the focus lies in front of the retina, or too 

 near, so that the focus lies behind the retina, and the pencils fall 

 on the retina not as points, but as systems of diffusion circles, the 

 sensation produced is that of an indistinct and blurred image. In 

 order that objects both near and distant may be seen with equal 

 distinctness by the same dioptric apparatus, the focal arrange- 

 ments of the apparatus must be accommodated or adjusted to the 

 distance of the object, either by changing the refractive power of the 

 lens, or by altering the distance between the lens and the screen. 



That the eye does possess such a power of accommodation or 

 adjustment is shewn by e very-day experience. If two needles be 

 fixed upright, some two feet or so apart, into a long piece of wood, 

 and the wood be held before the eye, so that the needles are nearly 

 in a line, it will be found that if attention be directed to the far 

 needle, the near one appears blurred and indistinct, and that, con- 



