Prof. Wheatstone on tha Physiology of Vision. 261 



joining the centres of the two eyes; the plane of the horopter 

 to be a plane passing through this line at right angles to that 

 of the optic axes. All objects which are in this plane, must, 

 according to him, appear single because the lines of dnection in 

 which any point of an object is seen coincide only in this plane 

 and nowhere else ; and as these lines can meet each other only 

 in one point, it follows from the hypothesis, that all objects not 

 in the plane of the horopter must appear double, because their 

 lines of direction intersect each other, either before or after they 

 pass through it. This opinion was also maintained byDechales 

 and Porterfield. That it is erroneous, I have given, I think, 

 sufficient proof, in showing that, when the optic axes converge 

 to any point, objects before or beyond the plane of the horopter 

 are under certain circumstances equally seen single as those in 

 that plane. 



Dr. Wells's " new theory of visible direction " was a modifi- 

 cation of the preceding hypothesis. This acute writer held with 

 Aguilonius, that objects are seen single only when they arc in 

 the plane of the horopter, and consequently that they appear 

 double when they are either before or beyond it; but he at- 

 tempted to make this single appearance of objects only in the 

 plane of the horopter to depend on other principles, from which 

 he deduced, contrary to Aguilonius, that the objects which ai'e 

 doubled do not appear in the plane of the horopter, but in other 

 places which are determined by these principles. Dr. Wells was 

 led to his new theory by a fact which he accidentally observed, 

 and which he coidd not reconcile with any existing theory of 

 visible direction; this fact had, though he was unaware of it, 

 been previously noticed by Dr. Smith ; it is already mentioned 

 in § 8, and is the only other instance of binocular vision of relief 

 which I have found recorded previous to my own investigations. 

 So little does Dr. Wells's theory appear to have been understood, 

 that no subsequent writer has attempted either to confirm or 

 disprove his opinions. It would be useless here to discuss the 

 principles of this theory, which was framed to account for an 

 anomalous individual fact, since it is inconsistent with the gene- 

 ral rules on which that fact has been now shown to depend. 

 Notwithstanding these erroneous views, the "essay upon single 

 vision with two eyes " contains many valuable experiments and 

 remarks, the truth of which arc independent of the theory they 

 wen: intended to illustrate. 



The theory which has obtained greatest currency is that which 

 assumes that an object is seen single because its pictures fall on 

 corresponding points of the two retina 1 , that is on points which 

 are similarly situated with respect to the two centres both in 

 distance and position. This theory supposes that the pictures 



