1873.] 



Dr. J. Jago on Visible Direction. 



217 



optic axes beyond them, they yield a hollow pyramid, and when looked at 

 by a previous decussation of these axes yield a solid pyramid. But these 

 perspectives are placed so that the one which was at the left has the 

 one that was at the right immediately underneath it, with about half an 

 inch of plain paper between them. 



Then it comes to pass that by properly displacing the right eyeball up- 

 wards, by means of the tip of the finger placed underneath it, we can put 

 the under perspective immediately upon the upper one seen with the 

 other eye, and thus realize the hollow pyramid ; or by placing the finger 

 upon the top of the left eye, we can depress the upper perspective to 

 cover the under one, and thus realize the solid pyramid. The first pyra- 

 mid depends from the plane of the paper, the second stands upon it. 



By means of a finger under one eye and another upon the other, we 

 can obtain either hollow or solid pyramids anywhere between the two 

 perspectives ; or by a finger on both eyes, or under both, we can obtain 

 the pyramid and its " converse" below or above both perspective outlines. 



In all these cases the optic axes do not intercept each other, but the 

 axes of visible direction (functional of the final directions of the optic 

 nerves) do meet on the paper — that is, the first pair of axes are not, and 

 the second pair are, in such a realization as is herein planned, in one plane. 



In all these cases the perspectives fall on similar parts of the two re- 

 tina), as in the modes originally mentioned by Wheatstone. 



The author goes on to consider in what way the sensorium refers the 

 sensations it receives notice of from the optic nerves into space, so as to fix 

 the place, size, and form of an object. 



The theory of vision in retinal normals being proved to be unten- 

 able, it is admitted that there is some such association of the two retina) 

 as to have fairly suggested the theory of "identical" or "covering" points ; 

 but this relation he believes to be subordinate to a law by which the sen- 

 sorium projects or emits its perceptions into space, as it were in two ima- 

 ginary cones of sight-rays, which, though not issuing from the ends of the 

 optic nerves, have apices whose positions are functional of the directions 

 of these ends for the instant in question ; and that it is by the intersection 

 of the sight-rays in these cones, limited by the law of similar retinal parts, 

 that the places, sizes, and forms of objects are determined. Hence if we 

 conceive that a pair of stereoscopic perspectives, one being imaged on 

 one retina and one on the other, exist as sight affections in miniature in 

 the substance of the optic nerves, the size of the resultant solid form will 

 be greater the greater is the distance from the nerves at which the axes 

 of visible direction intersect, or the optic axes when they are coincident 

 respectively with them. 



The paper concludes by exemplifying in sundry ways the modes in which 

 the conclusions in it may be applied in investigating seemingly anomalous 

 phenomena in physiological optics. 



VOL. XXI, 



