ON VISION. 355 



sighted or presbyopic eye, on the contrary, a convex lens is required, in 

 order to obtain a virtual image at a greater distance than the olyect ; and 

 it often happens that the rays must be made not only to diverge less than 

 before, but even to converge towards a focus behind such an eye, in order 

 to make its vision distinct. Presbyopic persons have in general a small 

 pupil, and, therefore, seldom acquire the habit of covering any part of it 

 with their eyelids. 



When the images of the same object fall on certain corresponding points 

 of the retina in each eye,* they appear to the sense only as one ; but if 

 they fall on parts not corresponding, the object appears double ;f and in 

 general, all objects at the same distance, in any one position of the eyes, 

 appear ajlke either double or single. The optical axes, or the directions 

 of the rays falling on the points of most perfect vision, naturally meet at 

 a great distance ; that is, they are nearly parallel to each other, and in 

 looking at a nearer object we make them converge towards it, wherever it 

 may be situated, by means of the external muscles of the eye ; while in 

 perfect eyes the refractive powers are altered, at the same time, by an 

 involuntary sympathy, so as to form a distinct image of an object at the 

 given distance. This correspondence of the situation of the axes with the 

 focal length is in most cases unalterable ; but some have perhaps a power 

 of deranging it in a slight degree, and in others the adjustment is imper- 

 fect : but the eyes seem to be in most persons inseparably connected toge- 

 ther with respect to the changes that their refractive powers undergo, 

 although it sometimes happens that those powers are originally very dif- 

 ferent in the opposite eyes. 



These motions enable us to judge pretty accurately, within certain 

 limits, of the distance of an object ; and beyond these limits, the degree 

 of distinctness or confusion of the image still continues to assist the judg- 

 ment. We estimate distances much less accurately with one eye than 

 with both, since we are deprived of the assistance usually afforded by the 

 relative situation of the optical axes ; thus we seldom succeed at once in 

 attempting to pass a finger or a hooked rod sideways through a ring, with 

 one eye shut. Our idea of distance is also usually regulated by a know- 

 ledge of the real magnitude of an object, while we observe its angular 

 magnitude ; and on the other hand a knowledge of the real or imaginary 

 distance of the object often directs our judgment of its actual magnitude. 

 The quantity of light intercepted by the air interposed, and the intensity 

 of the blue tint which it occasions, are also elements of our involuntary 

 calculation : hence, in a mist, the obscurity increases the apparent distance, 

 and consequently the supposed magnitude, of an unknown object. We 

 naturally observe, in estimating a distance, the number and extent of the 



* On corresponding points of the two retinae, see Newton, Op. Qu. 15. Wol- 

 laston, Ph. Tr. 1824. On single vision, see Le Clerc, Paris, 1679. Wells, Lond. 

 1791. Herholt, Kopenhag. 1814. Wollaston, Ph. Tr. 1824, p. 222. Twining, 

 Ed. Jour. ix. 143. 



f The most simple mode of witnessing this is to place a small wafer on a pane of 

 a window, and to look attentively through that pane at a well-defined object without 

 so as to fix the direction of the axes of the eyes. The spot will be distinctly 

 doubled. 



