On the Causes of Erect and Sirigle Vision. 4? 15 



cation of one object with that of another of a similar kind, placed 

 in a contrary direction to it. In the former case, two objects 

 do not present themselves, but only one of a kind, and that 

 surrounded by each and every line of demarcation, precisely 

 in the same relations to each other as are those of external 

 nature. The same observation holds good when drawings 

 are used with two images on them, placed contrariwise to each 

 other; as an arrow without the retina, and an arrow within 

 the retina. Did the arrow within the retina feel along with 

 the surrounding lines of colouring, there could be no sense of 

 an inverted arrow ; for there would exist no reference to an- 

 other arrow, which reference is only made by the observer, 

 who is looking on two arrows. 



Observations analogous to these must be made on the at- 

 tempted explanation " of the cause of single vision," by the 

 same author, who says*, " Because the lines of visible direc- 

 tion from similar points of the image (on one retina) meet the 

 lines of visible direction from similar points of the other image 

 upon the other retina, each pair of similar points must be 

 seen as one point." How so, when the mind sees not out of it- 

 self at the junction of the points, and when if the object which 

 sent forth the rays were annihilated, there would still result a 

 single vision from separated points of colour painted on sepa- 

 rated RETiNiE at a distance from each other; such duplicate 

 separate, figures on the retinae being the proximate cause of 

 the single vision of the object, and not the junction of similar 

 points, when rays are drawn back again from the retinas to such 

 points of junction. 



The question still recurs, and is still untouched and unex- 

 plained ; Why are pairs of points perceived by the mind as sin- 

 gle points ? No doubt the determination of rays upon the re- 

 tinae in such a manner that when drawn back again they will 

 meet at a central point, is a property closely connected with 

 the method of vision ; but it is rather a corollary or conse- 

 quence of the manner of the entry of the rays at the pupil oj the 

 eye by which equal arches are subtended upon the retina, than 

 the efficient cause of either single or erect vision. I again ask, 

 Why are two objects on the retinae perceived as only one ob- 

 ject by the mind? For it is not a junction of external points 

 which is perceived, but two sentient retinae determine two se- 

 parate images (equally perfect in their form, equally brilliant 

 in their colouring), as but one image to the mental capacity of 

 perception. Is not the answer, Because there are no points of 



* Optics, part ii. " Library of Useful Knowledge." 



colouring 



