CHAP, ii.] SIGHT. 493 



Accommodation. 



When an object, a lens, and a screen to receive the image, are 

 so arranged in reference to each other, that the image falls upon 

 the screen in exact focus, 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, subsequently diverg- 

 ing, will fall upon the screen as a circular patch composed of a 

 series of circles, the so-called diffusion circles, arranged concentri- 

 cally 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 arrangements of the apparatus 

 must be accommodated 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 is shewn 

 by every-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, conversely, when the near 

 one is distinct, the far one appears blurred. By an effort of the 

 will we can at pleasure make either the far one or the near one 

 distinct; but not both at the same time. When the eye is 

 arranged so that the far needle appears distinct, the image of 

 that needle falls exactly on the retina, and each pencil from each 

 luminous point of the needle unites in a point upon the retina; 

 but when this is the case, the focus of the near needle lies behind 

 the retina, and each pencil from each luminous point of this needle 

 falls upon the retina in a series of diffusion circles. Similarly, 

 when the eye is arranged so that the near needle is distinct, 



