SEC. 2. THE FACTS OF ACCOMMODATION. 



530. 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 lumi- 

 nous point of the object are brought into focus on the screen in 

 a point of the image corresponding t.o 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 from, bujb nearer to 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 arrangements 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 



846 



