MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 389 



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 re- 

 tinae, 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 projected on 

 the retinse are exactly similar to each other, corresponding points of the two pictures 

 falling on corresponding points of the two retinae. Authors who agree with regard 

 to this property, differ widely in explaining why objects are seen in the same place, 

 or single, according to this law. Dr. Smith makes it to depend entirely on custom, 

 and explains why the eyes are habitually directed towards an object so that its pic- 

 tures fall on corresponding parts in the following manner : — " When we view an ob- 

 ject steadily, we have acquired a habit of directing the optic axes to the point in view; 

 because its pictures falling upon the middle points of the retinas, are then distincter 

 than if they fell upon any other places ; and since the pictures of the whole object 

 are equal to one another, and are both inverted with respect to the optic axes, it fol- 

 lows that the pictures of any collateral point are painted upon corresponding points 

 of the retinas." 



Dr. Re ID, after a long dissertation on the subject, concludes, " that by an original 

 property of human eyes, objects painted upon the centres of the two retinae, or upon 

 points similarly situated with regard to the centres, appear in the same visible place; 

 that the most plausible attempts to account for this property of the eyes have been 

 unsuccessful ; and therefore, that it must be either a primary law of our constitution, 

 or the consequence of some more general law which is not yet discovered." 



Other writers who have admitted this principle have regarded it as arising from 

 anatomical structure and dependent on connexion of nervous fibres ; among these 

 stand the names of Galen, Dr. Briggs, Sir Isaac Newton, Rohault, Dr. Hartley, 

 Dr. WoLLASTON and Professor Muller. 



Many of the supporters of the theory of corresponding points have thought, or 

 rather have admitted, without thinking, that it was not inconsistent with the law of 

 Aguilonius ; but very little reflection will show that both cannot be maintained 

 together ; for corresponding lines of visible direction, that is, lines terminating in 

 corresponding points of the two retinae, cannot meet in the plane of the horopter 

 unless the optic axes be parallel, and the plane be at an infinite distance before the 

 eyes. Some of the modern German writers* have inquired what is the curve in which 

 objects appear single while the optic axes are directed to a given point, on the hypo- 

 thesis that objects are seen single only when they fall on corresponding points of the 

 two retinae. An elegant proposition has resulted from their investigations, which I 

 shall need no apology for introducing in this place, since it has not yet been men- 

 tioned in any English work. 



R and L (fig. 26.) are the two eyes ; C A, C A the optic axes converging to the 



* Tortual, die Sinne des Menschen. Munster, 1827. Bartels, Beitrage zur Physiologic der Gesichtssinnes. 

 Berlin, 1834. 



