Mr. S. Rowley on a New Theory of Vision. 205 



tance from the base of vision as that at which they would if these 

 images were directly seen in the axes. 



In this experiment with the papered wall, with every move- 

 ment of the head a movement of all the images in the same 

 direction takes place ; because with the eyes fixed toward any 

 two figures, a movement of the head in any direction will carry 

 in the same direction all the points of intersection of the lines 

 of direction, and consequently all the images in the two surfaces. 

 So when the axes are made to pass through any two of a series 

 of equal and equidistant objects to a point beyond, as in the 

 case of the experiment with the wafers upon glass, a move- 

 ment of the head in one direction will produce a movement of 

 the images in a contrary direction. 



There are other circumstances incident to a lateral image 

 in the surface of vision, besides distance from its twin, which 

 will produce habits operating to defeat a correct appreciation 

 of the two distances. 



Such a circumstance is the visible magnitude. 



Thus, whatever the visible magnitude of an aggreate lateral 

 image, which has been often seen in direct vision, this magni- 

 tude will be taken for that found on particular examination, 

 and the visible distance for that corresponding to that magni- 

 tude. But since lateral images in the surface of vision not 

 distinguishable from each other by consciousness may result 

 from objects of very different magnitudes placed at suitably 

 different distances, a given magnitude of image will not always 

 excite the same tendency of mind. 



To illustrate this, take the board with pins placed at A and 

 B (fig. 3), depress the far end till the surface is hidden by the 

 near end, and stopping the light flowing from the near pin to 

 one of the eyes by interposing near the eye an adequate object, 

 fix the axes of both toward the remote pin. As soon as the eye 

 has ceased its alternations of adjustment (which have been per- 

 formed so rapidly as to keep up a sensibly sustained distinct- 

 ness of both images), and no longer experiences the sensation 

 of focal adjustment to the near pin (which has excited a ten- 

 dency of mind to mistake the image at the distance of the ob- 

 ject), the now shadowy image of this pin will take its position 

 beside that of the far pin. This it will generally do slowly, be- 

 cause the image in the surface might have resulted from a pin 

 of due dimensions situated at every intermediate point, but in 

 continuous succession giving up every intermediate position, 

 it will finally come to rest in its true visible place. Occasionally, 

 owing to the dominancy of some habit, it will seem to take a 

 place at a greater or less distance than the true. 



