[From Cambridge PhUotophical Society Proceedings, VoL H. 365.] 



LXX. On the Centre of Motion of the Eye. 



THE aeries of positions which the eye assumes as it is rolled horizontally 

 have been investigated by Dondere (Donders and Doijer, Derde JaarUjlc.^/, 

 I'rrslag betr. het Nederlandsch Gasthuis voor Ooglijders. Utrecht, 1862), and 

 recently by Mr J. L. Tupper (Proc. R. S., June 18, 1874). The chief difficulty 

 in the investigation consists in fixing the head while the eyeball moves. 

 The only satisfactory method of obtaining a system of co-ordinates fixed with 

 reference to the skull is that adopted by Hehnholtz (Handbuch der Physiolo- 

 gixhen Optit, p. 517), and described in his Croonian Lecture. 



A piece of wood, part of the upper surface of which is covered with 

 warm sealingwax, is placed between the teeth and bitten hard till the sealing- 

 wax sets and forms a cast of the upper teeth. By inserting the teeth into 

 their proper holes in the sealingwax the piece of wood may at any time be 

 placed in a determinate position relatively to the skull. 



By this device of Helmholtz the patient is relieved from the pressure of 

 Hcrews and clamps applied to the skin of his head, and he becomes free 

 to move his head as he likes, provided he keeps the piece of wood between 

 his teeth. 



If we can now adjust another piece of wood so that it shall always have 

 a determinate position with respect to the eyeball, we may study the motion 

 >f the one piece of wood with respect to the other as the eye moves about. 



For this purpose a small mirror is fixed to a board, and a dot is marked 

 on the mirror. If the eye, looking straight at the image of its own pupil 

 in the mirror, sees the dot in the centre of the pupil, the normal to the 

 mirror through the dot is the visual axis of the eye a determinate line. 



A right-angled prism is fixed to the board near the eye in such a position 

 that the eye sees the image of its own cornea in profile by reflexion, first 



