214 



Dr. J. Jago on Visible Direction. 



[Mar. 13, 



priate mark on a grey wall, and finds that the spectrum really has its 

 margin across the point of direct sight ; and he tries other experiments 

 in corroboration. 



Also by agitating a pin-hole in a card across the eye when looking at 

 such a high object, he brings the retina into view, and sees that the point 

 of direct sight is visibly within the foramen centrale retina?, as made visible 

 by the shadow of the wall that bounds the foramen. He indicates other 

 means of proving the same fact. 



He gathers from a series of experiments that the mastery of the orbital 

 muscles over such movements of the eyeball as are requisite for pointing 

 the optic axis to its objective point, is practically unimpaired by such 

 shiftings of the eyeball in 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 negatively have in the punctum csecuin, 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. 



From these and other methodically continued experiments, he draws 

 the general inference, that if the centre of the foramen centrale retina? 

 be forced at any instant from its position by any sort of manipu- 

 lation, 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 direc- 

 tion (that is, has generated a cylinder in revolving), the axis of the 

 seeming field of vision will have so revolved 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. 



