Mr. S. Rowley on a New Theory of Vision. 193 



the other at a distance from this, say, f of an inch, and direct 

 his optic axis to the former, he will not see the two images of 

 the latter as one, though their object is situated in the plane 

 mentioned, but as two distinct images, the one beyond the 

 other. 



Experiments to the same effect might easily be multiplied ; 

 but the one given abundantly demonstrates the inaccuracy of 

 the theory of Aguilonius. 



The theory of corresponding points, so called (as held on phy- 

 siological grounds by Galen, Alhazen, and Newton), cannot be 

 denominated a theory of vision. It is directed to the accounting 

 for but one of the phenomena of vision, that of single sight with 

 two eyes. It does not deal with visible direction and distance, 

 whereas these conditions, governing, as they do, external visible 

 position, include the result of single sight with two eyes as a 

 necessary consequence. 



The theory of vision propounded by Dr. Smith in his e Optics/ 

 and that by Dr. Wells, from which he derives his explanation of 

 the phenomenon of single vision with two eyes in his ' Essay on 

 Single Vision with Two Eyes/ are practically (as I trust will 

 satisfactorily appear hereafter) merely attempts to formulate the 

 misapprehensions by consciousness of true visible distance which 

 take place under the influence of habit. A simple experiment 

 subversive of both these theories may be made thus. Let two 

 small objets, as pins in a board, be placed directly in front of the 

 middle point between the eyes, the one at some short distance, 

 say, 6 inches, and the other at some more considerable distance, 

 say, 18 inches, and let other two small objects be placed on op- 

 posite sides of the near one in .lines between the eyes and tfce 

 far object, and the optic axes be directed to the near object ; then 

 the two images of the remote object will be found at no greater 

 distance from each other than the images of the objects at the 

 sides of the near one, or all will be apprehended as lying in the 

 figure of a rectangle. 



But by the theory of Dr. Wells* the two images of the remote 

 object (since this object lies in the direction which he has named 

 the common axis) are seen in the optic axes, the right eye's image 

 in the left eye's axis, and vice versa, and at a distance from each 

 other (if located at the same distance from the eyes as the ob- 

 ject)! of 4y s inches, whereas the images of the two lateral near 

 objects, since these occupy the line named horopter, will be seen, 

 in the places of the objects, at a distance from each other of l--f> Q 

 inch J. 



As to the two images of any object more or less distant than 



* Essay upon Single Vision with Two Eyes, p. 38. London, 1818. 

 t Ibid, page 27. + Ibid, pages 5 and 46. 



Phil. Mag. S. 4. Vol. 36. No. 242. Sept. 1868. 



