264 SENSE OF SIGHT. 



Again, there is a point of approximation to the eye beyond which 

 objects cease to be distinctly seen, in consequence of the rays of light 

 striking so divergently upon the eye, that the focus falls behind the 

 retina. This point, too, varies according to the refractive power of 

 the eye ; and is, therefore, different in different individuals. In the 

 myopic or short-sighted, it is much nearer the eye than common ; in 

 the presbyopic or long-sighted, more distant. The iris here, again, 

 plays an important part, by its action in shutting off the most diverging 

 rays. 



There is also a limit beyond which objects are no longer visible. 

 This is owing to the Light from them becoming absorbed before it reaches 

 the retina, or so feeble as not to make the necessary impression. The 

 distance, consequently, at which an object may be seen, will depend 

 upon the sensibility of the retina, and partly on the colour of the ob- 

 ject ; a light colour being visible to a greater distance than a darker. 

 A distant object may also be imperceptible owing to the image, traced 

 on the retina, being too minute to be appreciated ; for the size of the 

 image diminishes as the distance of the object increases. The range 

 of distinct vision varies, likewise, with the individual, and especially 

 with the myopic and presbyopic ; and in such case the pupil dilates to 

 admit as much light as possible into the interior of the eye, and to com- 

 pensate in some measure for the defect. 



Between the ranges of distant and near vision, a thousand different 

 examples occur. In all cases, however, the ocular cone must be brought 

 to a focus on the retina, otherwise there cannot be perfect vision. It 

 has been already observed, in the proem on light, that the distance, at 

 which the ocular cone arrives at a focus behind the lens, is in propor- 

 tion to the length of the objective cone ; or, in other words, that the 

 focus of a lens varies with the distance at which a radiant point is 

 situate before it : where the point is near the lens the focus will be 

 more remote behind it ; and the contrary. If this occurs in the human 

 eye it must necessarily follow ; either that it is not necessary for 

 an object to be impressed upon the retina ; or that the eye is capa- 

 ble of accommodating itself to distances ; or if it does not occur, it 

 must be admitted, that, owing to the particular constitution of the eye, 

 the impressions are so made on the retina as not to need such adapta- 

 tion. The whole bent of the foregoing observations on vision would 

 preclude the admission of the first of these postulates. The second has 

 been of almost universal reception, and given rise to many ingenious 

 speculations ; and the third has been seriously urged of late years only. 



It would occupy too much space to dwell at length upon the various 

 ingenious discussions, and the many interesting and curious experiments, 

 that have resulted from a belief in the power possessed by the eye of 

 accommodating itself to distances. It is a subject, however, which 

 occupies so large a field in the history of physiological opinions, that 

 it cannot be passed over. The chief views, that have been entertained, 

 are : First. The cornea or lens must recede from, or approach the 

 retina, according to the focal distance, precisely as we adapt our tele- 

 scopes by lengthening or shortening the tube. Secondly. If we suppose 

 the retina to be stationary, the lens must experience a change in its 



