194 Prof. J. LeConte on some Phenomena 



own images. When we look at any object, we bring the two 

 external images of that object into coincidence at the point of 

 sight. Now the point of sight, together with all other corre- 

 sponding points of the two fields of view which coalesce at that 

 moment, constitute the horopter. Of course the images of all 

 points lying in the horopter fall on corresponding points of the 

 retina. 



Is the horopter a surface or is it a line ? In either case what 

 is its form and position ? These questions have tasked the inge- 

 nuity of physicists, mathematicians, and physiologists. If the 

 position of identical points of the retinae under all circumstances 

 were known, then the question of the form of the horopter would 

 become a purely mathematical one. But the position of identical 

 points evidently depends upon the laws of ocular motion. It is 

 evident, therefore, that it is only on an experimental basis that a 

 true theory of the horopter can be constructed ; and yet the ex- 

 perimental investigation as usually conducted is very unsatisfac- 

 tory, on account of the indistinctness of vision when the object 

 is at any considerable distance from the point of sight in any 

 direction. 



The most diverse views have, therefore, been held as to the 

 nature and form of the horopter. Aguilonius, the inventor of 

 the name, believed it to be a plane passing through the point of 

 sight and perpendicular to the median line of sight. Others 

 have believed it to be the surface of a sphere passing through the 

 point of sight and the optic centres ; others, a torus formed by 

 the revolution of a circle passing through the point of sight and 

 the optic centres on a line joining the optic centres. The sub- 

 ject has been investigated with great acuteness by P. Prevost, A. 

 Prevost, J. Muller, G. Meissner, E. Claparede*, and, lastly, by 

 Helmholtz f. A. Prevost determines in it, as he supposes, a circle 

 passing through the optic centres and the point of sight, which 

 he calls the " horopteric circle/ 3 and a straight line passing 

 through the point of sight at right angles to the visual plane, 

 which he calls the " horopteric vertical." 



Until the investigations of Meissner, almost all attempts to 

 determine the form of the horopter have been by mathematical 

 calculations, based upon the doctrine of identical points, and 

 assuming the law of Listing. Meissner attempts the same ques- 

 tion experimentally. We condense the following account of his 

 admirable investigations from Claparede's memoir on this sub- 

 ject J already referred to. 



* Bib. Un. Archiv. des Scien. II. vol. iii. pp. 138 & 225. 



t Proc. Roy. Soc. April 1864. 



X Bib. Un. Arch, des Scien. II. vol. iii. p. 138. 



