THE SENSES. 713 



object may be considered as a series of points, from each of which a 

 pencil of light diverges to the eye, and this pencil has for its centre or 

 axis, a ray which impinging upon the refractive surface perpendicularly 

 to the surface is not refracted, but passes through the nodal point, and 

 is prolonged backward to the retina, whereas the diverging rays are also 

 made to converge to a principal posterior focus behind the lens, or the 

 chief axis of the pencil of light proceeding from the point in question, 

 and this focus, if the image is to be clear, should fall on the retina. 



Thus from each point of an object a corresponding image is formed 

 on the retina, so that an image of the distal object is produced. It is 

 an inverted image. Whether the image is blurred or not depends upon 

 the refractive power of the media, and upon the distance of the anterior 

 surface of the cornea from the retina. If the refractive media are too 

 powerful, or the eye too long, the image is formed in front of the retina 

 (fig. 422) ; if the reverse, the image is formed behind the retina, and in 

 both cases an indistinct and blurred image is the result. 



ACCOMMODATION. 



The distinctness of the image formed upon the retina, is mainly de- 

 pendent on the rays emitted by each luminous point of the object being 

 brought to a perfect focus upon the retina. If this focus occur at a 

 point either in front of, or behind the retina, indistinctness of vision 

 ensues, in the way we have already described, with the production of a 

 halo. The focal distance, i.e., the distance from a lens of the point at 

 which the luminous rays are collected, besides being regulated by the 

 degree of convexity and density of the lens, varies with the distance of 

 the object from the lens, being greater as this is shorter, and vice versa. 

 Hence, since objects placed at various distances from the eye can within 

 a certain range, different in different persons, be seen with almost equal 

 distinctness, there must be some provision by which the eye is enabled to 

 adapt itself, so that whatever length the focal distance may be, the focal 

 point may always fall exactly upon the retina. 



This power of accommodation, or the adaptation of the eye to vision 

 at different distances, has received the most varied explanations. It is 

 obvious that the effect might be produced in either of two ways, viz., (a) 

 by altering the convexity, and thus the refracting power, either of the 

 cornea or of the lens; or (b) by changing the position either of the 

 retina or of the lens, so that whether the object be near or distant, the 

 focal points to which the rays are converged by the lens may always fall 

 exactly on the retina. The amount of either of these changes, which 

 would be required in even the widest range of yision, would be extremely 

 small. For, from the refractive powers of the media ot tfea eye,, 1tfee dif- 



