i84 THEPHILOSOPHY 



\y true, that, when one eye is diftorted by the finger, or any other 

 caufe, in fuch a manner that the images are painted on points of the 

 retinae which do not correfpond, the objed is perceived to be 

 double. Obje£ls which are much nearer, or much more remote, 

 than that to which both eyes are diredted, appear double. If a 

 candle is placed at the diftance of ten feet, and a man holds his 

 finger at arm's-length between his eyes and the candle, when he 

 looks at the candle, he fees his finger double, and, when he looks at 

 his finger, he fees the candle double. ' In this phaenomenon,' 

 Dr Reid properly remarks, * it is evident to thofe who underftand 

 ' optics, that the pictures of objeds which are feen double, do not 

 ' fall upon points of the retinae which are fimilarly fituated, but 

 ' that the pidures of objedts feen fingle do fall upon points fimilar- 

 ' ly fituated. Whence we infer, that as the points of the two reti- 

 ' nae, which are fimilarly fituated with regard to the centres, do 

 ' correfpond, fo thofe which are diffimilarly fituated do not corre- 

 ' fpond. It is to be obferved, that although, in fuch cafes as are 

 * mentioned in the laft phaenomenon, we have been accuftomed 

 ' from infancy to fee objeds double which we know to be fingle ; 

 ' yet cuftom, and experience of the unity of the objedt, never take 

 ' away this appearance of duplicity *.' 



The fenfe of feeing, without the aid of experience, conveys no 

 idea of diftance. If not aflifted by the fenfe of touching, all objedts 

 would feem to be in contadt with the eye itfelf. Objeds appear 

 larger or fmaller according as they approach or recede from the eye, 

 or according to the angle they fubtend. A fly, when very near the 

 eye, feems to be larger than a horfe or an ox at a diftance. Children 

 can have no idea of the relative magnitude of objeds, becaufe they 

 have no notion of the different diftances at which they are feen. It 



is 



* Dr ReiJ's Inquiry, &c. page 287. 



