ON VISION. 453 



When the imao-es 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; and in general, 

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

 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 adjust- 

 ment is imperfect: but the eyes seem to be in most persons inseparably con- 

 nected together with respect to the changes that their refractive powers 

 undergo, although it sometimes happens that those powers are originally very 

 different 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 distinct- 

 ness or confusion of the image still continues to assist the judgment. 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 knowledge of the real magni- 

 tude 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 intervening objects ; so that a distant church 

 in a woody and hilly country appears more remote than if it were situated 

 in a plain; and for a similar reason, the apparent distance of an object seen 



