270 SENSE OF SIGHT. 



be rectified. Sir Charles Bell 1 conceives, "that the mechanism of the 

 eye has not so great a power of adapting the eye to various distances 

 as is generally imagined, and that much of the effect, attributed to 

 mechanical powers, is the consequence of the motion of the pupil and 

 the effect of light and of attention. An object looked upon, if not 

 attended to, conveys no sensation to the mind. If one eye is weaker 

 than the other, the object of the stronger eye alone is attended to, and 

 the other is entirely neglected: if we look through a glass with one 

 eye, the vision with the other is not attended to." "The mind," he 

 adds, "not the eye, harmonizes with the state of sensation, brightening 

 the objects to which we attend. In looking on a picture or panorama, 

 we look to the figures, and neglect the background; or we look to the 

 general landscape, and do not perceive the near objects. It cannot be 

 an adaptation of the eye, but an accommodation, and association of the 

 mind with the state of the impression." 



The view, which we have expressed upon the subject, is confirmed 

 by the calculations of different investigators. From the refractive 

 powers of the different media of the eye it- was calculated by Olbers, 

 that the difference between the focal distances of the images of an 

 object at such distance that the rays are parallel, and of one at the" 

 distance of four inches, is only about 0.143 of an inch; so that the 

 change in the distance of the retina from the lens required for vision 

 at all distances, supposing the cornea and lens to maintain the same 

 form, would not be more than about a line. Again: M. de Simonoff, 2 

 a learned Russian astronomer, asserts, that from a distance of four 

 inches to infinity the changes in the angle of refraction are so small 

 that the apices of luminous cones, in a properly formed eye, must 

 always fall within the substance of the retina; and hence no variation 

 in the shape of the eye, according to the distance of the object, can be 

 necessary. Such facts amply justify the interrogatory of M. Biot; 3 

 whether the aberration of the focus for different distances may not be 

 compensated in the eye by the intimate composition of the refractive 

 bodies, as the aberration of sphericity probably is? Yet, if this be the 

 case, how admirable must be the construction of such an instrument ! 

 how far surpassing any effort of human ingenuity ! an instrument capa- 

 ble of not only correcting its own aberrations of sphericity, and refran- 

 gibility, but of seeing at all distances. 4 



It has been before observed, that the visual point varies in different 

 individuals. As an average, it may be assumed at eight inches from 

 the eye. There are many, however, who, either from original confor- 

 mation of the organ, or from the progress of age, wander largely from 

 this average; the two extremes constituting myo-py or short-sightedness, 

 and presbyopy or long-sightedness. 



1 Anat. and Physiology, edit, cit., ii. 230. 



2 Magendie's Journal de Physiologic, torn. iv. and Precis de Physiol., i. 73. 



3 Traite de Physiologic Experimental, Paris, 1816. 



4 Letters of Euler, by Sir D. Brewster, Amer. edit., i. 163, New York, 1833. See, on 

 this subject, Volkmann, Art. Sehen, in Wagner's Haridworterbuch der Physiologie, 14te 

 Lieferurig, s. 295, Braunschweig, 1846; and Bily and Kirkes, Recent Advances in the Phy- 

 siology of Motion, the Senses, Generation and Development, p. 20, Lond., 1848. 



