746 SIGHT. 



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 corre- 

 sponding to the point of the object. If the object be then removed further 

 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 

 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 further from, but 

 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 to the distance of the object, either by changing the refrac- 

 tive power of the lens or by altering the distance between the lens and the 

 screen. 



620. That the eye does possess such a power of accommodation is shown 

 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, the image of 

 that needle falls upon the retina in such a way, that each pencil of rays 

 from each luminous point of the needle unites in a point on the retina, while 

 each pencil from each luminous point of the far needle unites at a point in 

 front of the retina, and then diverging again falls on the retina in a series 

 of diffusion circles. If the near needle be gradually brought nearer and 

 nearer to the eye, it will be found that greater and greater effort is required 

 to see it distinctly, and at last a point is reached at which no effort can make 

 the image of the needle appear anything but blurred. The distance of this 

 point from the eye marks the limit of accommodation for near objects. 

 Similarly, if the person be short-sighted, the far needle may be moved 

 away from the eye, until a point is reached at which it ceases to be seen 

 distinctly, and appears blurred. In the one case the eye, with all its power, 

 is unable to bring the image of the needle sufficiently forward to fall on the 

 retina ; the focus lies permanently behind the retina. In the other the eye 

 cannot bring the image sufficiently backward to fall on the retina ; the focus 

 lies permanently in front of the retina. In both cases the pencils of rays 

 from the needles strike the retina in diffusion circles. 



621. The same phenomena may be shown with greater nicety by 



