270 ON SIE DAVID BBEWSTEE'S SUPPOSED 



" cone of rays variously inclined from 85° to 90° to the surface of 

 " the retina. While the point is distinctly seen, intercept all these 

 " different rays in succession, and it will be found that each ray gives 

 " vision in the same direction, the visible point retaining its position. 

 " Hence it follows, that on the part of the retina in the axis of vision, 

 " all rays, however obliquely incident, give the same visible direction 

 " perpendicular to the surface of the membrane." Now, I admit 

 that a very interesting fact in vision is here proved : and let Sir 

 David Brewster have the credit of having established it. But what 

 is the fact proved ? It is, that all rays falling upon the part of the 

 retina which lies in the axis of vision, give rise to the same subjective 

 affections, whatever be the inclination at which they impinge upon 

 the retina. Nothing else than this is made out. Sir David 

 Brewster indeed thinks, that, because the visible point retains its 

 position while the different rays in succession are intercepted, we are 

 warranted in affirming that " each ray gives visible position in the 

 same direction." But what is meant by the visible point retaining 

 its position? There does not exist any visible point, or image, to 

 which position in absolute space, apart from the mind, can be ascribed. 

 When a visible point, therefore, is said (popularly) to retain its posi- 

 tion, the idea really intended to be conveyed, must be, that no 

 appreciable alteration is experienced in the subjective affections of 

 which we are conscious. If we refer (as we are under no necessity 

 of doing) our subjective affections to a remote stimulus, it is of 

 course to be expected, that, while no appreciable change takes place 

 in the subjective affections, no wavering or variation shall occur in 

 the estimate which the mind forms of the direction of the stimulus. 

 But the circumstance of " the visible point retaining its position" 

 indicates nothing whatever jegarding such reference, whether deter- 

 minate or variable. A visible point is a subjective phenomenon. A 

 change in its position is a change occurring in a subjective sphere. 

 The absence of any change in its position is the absence of (appre- 

 ciable) change, in a certain respect, in our subjective affections. The 

 experiment described merely shews, therefore, that all rays falling 

 upon the part of the retina in the axis of vision give rise to the same 

 subjective affections ; and hence it has no weight in demonstrating 

 the law in support of which it is adduced. 



But besides failing to observe that the circumstance of the 

 visible point retaining its position while the different rays in succes- 

 sion are intercepted, amounts to no more than this, that rays incident 

 upon the same part of the retina at different obliquities give rise to 



