'Royal Society. 81 



unimpaired by such shiftings of the eyeball iu its socket as have 

 been described. 



He then proceeds to show that the regulating duties of the 

 orbital muscles, when the eyeball is displaced in its orbit, are not 

 only fulfilled as to the rotation of the optic axis about a central 

 point, but as to the rotation of the eyeball about this axis. To 

 make experiments to this end, we must have another subjectively 

 visible retinal spot besides the foramen centrale ; and this we nega- 

 tively have in the punctum csecum, or at the base of the optic nerve. 



A diagram is devised by which we may manage that one point 

 of it shall be seen by the direct sight of both eyes, whilst another 

 point is found to fall in the middle of the blind spot of one eye ; 

 and the diagram is examined by this eye when this has been pushed 

 from its orbital place upwards, downwards, inwards, and outwards, 

 and in various oblique directions, besides when more or less twisted 

 on its axis ; and thus it is demonstrated that what happened with 

 the point of direct sight in the retina happens equally surely for 

 every other retinal point — that under all these displacements the 

 orbital muscles do not forfeit their control of the eyeball, but so 

 regulate its movements that the different points of the field of vision 

 remain constantly painted on the same retinal points. 



Prom these and other methodically continued experiments, he 

 draws the general inference, that if the centre of the foramen cen- 

 trale retina3 be forced at any instant from its position by any sort 

 of manipulation, and then made to describe a circle round its first 

 position of ease whilst the optic axis has never ceased to remain 

 parallel to its first direction (that is, has generated a cylinder in 

 •revolving), the axis of the seeming field of vision will have so re- 

 volved as to have generated a cone, whose apex is posterior to the 

 retina in the first or undisturbed direction of the optic axis. The 

 like might have been said of any other normal to the retina, the 

 axis of the base of the optic nerve, for instance, were it accessible to 

 light, whilst a twisting retinal movement about a fixed axis twists 

 the seeming field of vision. 



If the optic axis revolve so as to generate a cone whose apex is in 

 front of the eye, the axis of the seeming field may, according to 

 circumstances, generate a cylinder, or a more acute cone enclosing 

 the other. 



Conversely, the parallax of the visual field being noted, we can 

 assign the retinal displacements that have produced them. 



Should undue contraction of any orbital muscle, or discordant 

 contractions of the orbital muscles, engender visual parallaxes, we 

 may as safely judge from these parallaxes of the retinal displace- 

 ments that must have been induced, as if they had been due to 

 manipulation of the eyeball. 



In these summarized conclusions we have the means of solving 

 highly important problems in physiological optics. 



It is found that sensation, or the function of responding to ob- 

 jective light, is exclusively resident in the retinal elements of the 

 bncillar layer, but that the visual functions of the retina extend no 



Phil Mag. S. 4. Vol. 46. No. 303. July 1873. G 



