262 Prof. Wheatstone on the Physiology of Vision. 



projected on the retinae are exactly similar to each other, corre- 

 sponding 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 pictures 

 fall on corresponding parts in the following manner : — " When 

 we view an object 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 follows that the pictures of any 

 collateral point are painted upon corresponding points of the 

 retinas." 



Dr. Reid, after a long dissertation on the subject, concludes, 

 " that by an original property of human eyes, objects painted 

 upon the centres of the two retina?, 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 pro- 

 perty of the eyes have been unsuccessful ; and therefore, that it 

 must be either a primary law of our constitution, or the conse- 

 quence 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 con- 

 nexion of nervous fibres ; among these stand the names of Galen, 

 Dr. Briggs, Sir Isaac Newton, Rohault, Dr. Hartley, Dr. Wol- 

 laston and Professor Muller. 



Many of the supporters of the theory of corresponding points 

 have thought, or rather have admitted, ivithout 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 termi- 

 nating in corresponding points of the two retinae, cannot all at 

 the same time 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 hypothesis that objects are 

 seen single only when they fall on corresponding points of the 

 two retinae. An elegant proposition has resulted from their inves- 

 tigations, which I shall need no apology for introducing in this 

 place, since it has not yet been mentioned in any English work. 



* Tortual, die Shine des Menschen. Minister, 1827. Bartels, Beitrage 

 zur Physiologie der Gesichtssines. Berlin, 1834. 



