1874.] Mr. J . L. Tupper on the Centre of Motion in the Eye. 429 



XIII. " On the Centre of Motion in the Human Eye." By J. L. 

 TUPPER. Communicated by S. J. A. SALTER, F.R.S. Received 



May 15, 1874. 



(Abstract.) 



The paper of which this is a short abstract premises that its argu- 

 ment is conditional, that it adopts all the fundamental optical conditions 

 as they are received, that the received centre of motion is not one of 

 these, but is supposed to be legitimately derived from them, and that the 

 author disputes this and proposes : 



1st. To show that this conclusion is inconsistent with its premises, 

 and that a different though indefinite conclusion is thence derivable ; 



2nd. By experiment, to develop and reduce that conclusion to a de- 

 finite form ; 



3rd. To verify it by anatomical induction. 



The latest investigations (those of Prof. Donders) have placed the 

 centre of motion nearly two millimetres behind the centre of the globe, 

 and in the cornea's axis. The process of proof assumed that the centre 

 of motion is equidistant from the outer and inner margins of the cornea, 

 and, moreover, that the eye's visual line (ordinarily at 6 with the 

 cornea's axis) will, by mere rotation, in turn coincide with three or more 

 radii of the same circle ; or that, without moving the head, we can suc- 

 cessively sight the lines on a graduated circular arc, seeing them as so 

 many points. 



The paper first proves, by a geometrical diagram, that if the eye, by 

 simple rotation, can thus see the radii of a circle, the centre of motion 

 must be in the visual line, not in the cornea's axis, as hitherto supposed ; 

 proves next, by pairs of sights set up on the radii of a circle, and actually 

 seen as so many points, that the centre of motion is, in fact, in the visual 

 line ; and proves, lastly, by measuring (mechanically) how far the front 

 of the cornea is from the converging point of the radii thus sighted, that 

 the centre of motion is about Jt- of an inch, instead of ^|- of an inch, be- 

 hind the cornea's anterior surface. 



Then follows a twofold anatomical corroboration of these conclusions 

 by examination, 



1st, of the living eye ; 



2nd, of the dissected eye. 



(1st) If the eye rotated on a point in the antero-posterior diameter (or 

 cornea's axis), then any two points equidistant from the cornea's centre 

 would in turn occupy the same point in space, as assumed by Prof. 

 Donders. The first experiment shows that two such corresponding points 

 will not, as the eye turns, fall into the same place ; whilst other examina- 

 tions of the living eye show not only that symmetrically situated points 

 move asymmetrically, but move asymmetrically in such a way as would 

 occur if the centre of motion were external to the antero-poaterior axis, 



