410 On the Causes of Erect and Single Vision. 



countries could arise, so long as the surfaces were regulated 

 to the same degree of latitude and longitude; but the moment 

 they were separated by an alteration of the longitude of either, 

 or both, the sense of that newly arisen continent, or sea, would 

 divide the sense of the remainder. 



The following passage is extracted from the Encyclopedia 

 Britannica, and is a quotation from Dr. Wells's Essay on 

 Single Vision. " If the question be concerning an object at 

 the concourse of the optic axes, it is seen single ; because its 

 two similar appearances in regard to shape, size, and colour, 

 coincide with each other through the whole of their extent." 



This opinion thus expressed, comes nearer to my meaning 

 than any other with which I am acquainted ; nevertheless Dr. 

 Wells's argument on the subject (and which is too long to 

 insert here,) is as fallacious as that of any of his predecessors; 

 inasmuch as it assumes an hypothetical law of vision, in order 

 to establish that coincidence of shape, size, and colour upon 

 which he perceived single vision did necessarily depend. 



This laborious argument of his, is as entirely needless as 

 it is futile ; because it proceeds upon the supposition that 

 objects are seen by the mind, beyond the mind, at the angle formed 

 by the axes of the eyes, in their direction to the same point 

 of distance. 



Now when, on the one hand, colour (conscious visible 

 colour) is admitted to be in the mind, and never to proceed 

 again out of it, in any line, or at any angle to form an ob- 

 ject ; and on the other hand, by those demonstrable laws of 

 optics established by Sir I. Newton and others, "that when 

 the axes of the eyes are directed to a given point or figure, 

 the said figure is painted on corresponding points of the re- 

 tinae," — then the coincidence which Dr. Wells speaks of, 

 must of necessity take place ; and such coincidence can only 

 determine a consciousness of single vision to the mind ; or in 

 other words, can only determine those similar appearances of 

 colour, on which visible size and figure ultimately and alone 

 depend : for the centres coincide with the centres, and the 

 edges with the edges, of the figures ; without any variety, or 

 interval of colouring between the interior and horizontal edges, 

 — the conscious sense of which is absolutely necessary in order 

 to induce a sense of variety or plurality of figure. When the 

 figure is painted upon points of the retinae which do not cor- 

 respond, then there must necessarily arise a sense of 'two figures ; 

 because the centres not coinciding with the centres, nor the 

 edges with the edges, there exists a surplus colour in one eye 

 which divides the interior and horizontal edges of the two 

 figures. This surplus colouring is determined to the retina 



by 



