260 Prof. Wheatstone on the Physiology of Vision. 



§15. 



No question relating to vision has been so much debated as 

 the cause of the single appearance of objects seen by both eyes. 

 I shall in the present section give a slight review of the various 

 theories which have been advanced by philosophers to account 

 for this phenomenon, in order that the remarks I have to make 

 in the succeeding section may be properly understood. 



The law of visible direction for monocular vision has been 

 variously stated by different optical writers. Some have main- 

 tained with Drs. lleid and Porterfield, that every external point 

 is seen in the direction of a line passing from its picture on the 

 retina through the centre of the eye; while others have sup- 

 posed with Dr. Smith that the visible direction of an object 

 coincides with the visual ray, or the principal ray of the pencil 

 which flows from it to the eye. D'Alembert, furnished with 

 imperfect data respecting the refractive densities of the humours 

 of the eye, calculated that the apparent magnitudes of objects 

 would differ widely on the two suppositions, and concluded that 

 the visible point of an object was not seen in either of these di- 

 rections, but sensibly in the direction of a line joining the point 

 itself and its image on the retina ; but he acknowledged that he 

 could assign no reason for this law. Sir David Brewster, pi*o- 

 vided with more accurate data, has shown that these three lines 

 so nearly coincide with each other, that " at an inclination of 

 30°, a line perpendicular to the point of impression on the retina 

 passes through the common centre, and does not deviate from 

 the real line of visible direction more than half a degree, a quan- 

 tity too small to interfere with the purposes of vision." We 

 may, therefore, assume in all our future reasonings the truth of 

 the following definition given by this eminent philosopher : — "As 

 the interior eye-ball is as nearly as possible a perfect sphere, lines 

 perpendicular to the surface of the retina must all pass through 

 one single point, namely the centre of its spherical surface. This 

 one point may be called the centre of visible direction, because 

 every point of a visible object will be seen in the direction of a 

 line drawn from this centre to the visible point." 



It is obvious, that the result of any attempt to explain the 

 single appearance of objects to both eyes, or, in other words, the 

 law of visible position for binocular vision, ought to contain 

 nothing inconsistent with the law of visible direction for mono- 

 cular vision. 



It was the opinion of Aguilonius, that all objects seen at the 

 same glance with both eyes appear to be in the plane of the ho- 

 ropter. The horopter he defines to be a line drawn through the 

 point of intersection of the outic axes, and parallel to the line 



