132 SENSES. 



jects single with two eyes. He tells us, that though a dis- 

 tinct image is painted on each retina, whenever these images 

 are painted on corresponding points of the retinae, an object 

 is perceived to be single. It is equally true, that, when one 

 eye is distorted by the finger, or any other cause, in such a 

 manner that the images are painted on points of the retinae 

 which do not correspond, the object is perceived to be double. 

 Objects which are much nearer, or much more remcte 

 than that to which both eyes are directed, appear doulle 

 If a candle is placed at the distance of ten feet, and a man 

 holds his finger at, arm's length between his eyes and the can- 

 dle, when he looks at the candle, he sees his finger double, 

 and when he looks aJ; his finger, he sees the candle double. 

 ' In this phenomenon/ Dr. Reid properly remarks, ' it is 

 evident to those who understand optics, that the pictures of 

 objects which are seen double, do not fall upon the points of 

 the retina? which are similarly situated, but that the pictures 

 of objects seen singly do fall upon points similarly situated. 

 Whence we infer that as the points of the two retinae, which 

 are similarly situated with regard to the centres, do corre- 

 spond, so those which are dissimilarly situated, do not corre- 

 spond. It is to be observed, that although, in such cases as 

 are mentioned in the last phenomenon, we have been accus- 

 tomed from infancy to see objects double, which we know to 

 be single, yet custom and experience of the unity of the object 

 never take away this appearance of duplicity.' * 



The sense of seeing, without the aid of experience, conveys 

 no idea of distance. If not assisted by the sense of touching, 

 all objects would seem to be in contact with the eye itself. 

 Objects appear larger or smaller according as they approach 

 or recede from the eye, or according to the angle they sub- 

 tend. A fly, when very near the eye, seems to be larger than 

 a horse or an ox at a distance. Children can have no idea 

 of the relative magnitude of objects, because they have no 

 notion of the different distances at which they are seen. It 

 is only after measuring space by extending the hand, or oy 

 transporting their bodies from one place to another, that cl il- 

 dren acquire just ideas concerning the real distances and 

 magnitudes of objects. Their ideas of magnitude result en- 

 tirely from the angle formed by the extreme rays reflected 

 from the superior and inferior parts of the object : hence every 

 near object must appear to be large, and every distant one 

 small. But after having, by touch, acquired ideas of distances, 



* Dr. Reid's Inquiry, &c. p. 287. S 



