Dr. J. Jago on Visible Direction. 83 



would deviate in the same direction, and an equal deviation in 

 visible direction would be associated with every other point in the 

 visual field. 



Hence we are fairly landed upon the conclusion that visible di- 

 rection, which has already been tracked backwards to the optic nerve, 

 is a function of its terminal direction, being identical with it at the 

 centre of the optic disk, both in the equable use of the eye and in 

 the unequable. 



Finally, it is clear that if the eyeball be twisted round the axis 

 of the optic disk the terminal portion of the nerve will be twisted 

 in the same direction; and thus the opposite twisting of the visible 

 field in certain experiments related are explicable by the same hy- 

 pothesis — an hypothesis that accounts for all the phenomena of 

 visible direction, whether regular or irregular. 



Whenever the inverted retinal image, by means of nervous arrange- 

 ment, is ^inverted, an erect image is seemingly projected, if not 

 from, by means of the base of the optic nerve. 



The principles here arrived at, when applied to binocular vision, 

 lead to the observation of phenomena that have not been before 

 put on record. 



Wheatstone, in his classic paper in the Philosophical Transac- 

 tions, wherein he announces his discovery of the stereoscope and 

 expounds its theory, only speaks of stereoscopic vision from two 

 perspectives, an appropriate one for each eye, when (no instrument 

 being used) the optic axes meet each other beyond them, or have pre- 

 viously intersected, so that each eye sees the other's perspective ; 

 that is, in all experiments by him and other subsequent writers on 

 the subject the optic axes have always been supposed to intersect or 

 to he in one plane. 



But as it has been demonstrated that the axes of visible direction 

 need not be coincident with the optic axes, it ought to follow that 

 we may continue to see bodies in relief from a pair of stereoscopic 

 perspectives, though these are not placed transversely to each other. 



Two perspectives of a pyramid are drawn, such as, when placed 

 laterally apart as is usual in stereoscopic slides, and looked at by 

 concourse of the optic axes beyond them, 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 im- 

 mediately underneath it, with about half an inch of plain paper 

 between them. 



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

 ball upwards, 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 de- 

 press the upper perspective to cover the under one, and thus realize 

 the solid pyramid. The first pyramid 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. 



G2 



