Dr. Brewster on the Vision of Impressions on the Retina. 7 



Dr. Knox, that he saw a case of a protrusion of the iris through 

 the cornea, which could very readily be distinguished even 

 when the eyelids were closed ; and that the protuberance occiu- 

 pied the same position whether the eyelids were open or shut. 



The impossibility of the existence of such a motion may 

 be deduced also from other principles. When the observer, 

 with a spectrum in his eye, closes his eyelids, Mr. Bell admits 

 that the spectrum remains stationary, which is undoubtedly 

 the case ; but as we have already demonstrated that the spec- 

 trum actually follows the movements of the eye as it ought 

 to do, upon the ordinary principles of optics, the absolute 

 immobility of the impression, upon shutting the eyelids, be- 

 comes an incontrovertible proof, that when the eye is closed, 

 the eyeball is pot displaced by the action of any involuntary 

 muscles. 



In order to strengthen his arguments for the existence of 

 this involuntary revolution of the eyeball, Mr. Bell has statr 

 ed, in a very ingenious manner, the final cause of such an ar- 

 rangement. 



M The purpose of this rapid insensible motion of the eyeball will be 

 understood on observing the form of the eyelids, and the place of the 

 lachrymal gland. The margins of the eyelids are flat, and when they 

 meet, they touch only at their outer edges, so that when closed, there is a 

 gutter left between them and the cornea. If the eyeball were to remain 

 without motion, the margins of the eyelids would meet in such a man- 

 ner on the surface of the cornea, that a certain portion would be left 

 untouched, and the eye would have no power of clearing off what obscur-? 

 ed the vision, at that principal part of the lucid cornea, which is in the 

 very axis of the eye ; and if the tears flowed, they would be left accumu- 

 lated on the centre of the cornea ; and winking, instead of clearing the 

 eye, would suffuse it. To avoid these effects, and to sweep and clear the 

 eurface of the cornea, at the same time that the eyelids are closed, the 

 eyeball revolves, and the cornea is rapidly elevated under the eyelid."-!- 

 Phil. Trans. 1823, p. 169. 



Unfortunately for these views, the clearing away of the 

 lubricating fluid which is left in the groove between the closed 

 eyelids has not been accomplished by Almighty wisdom. 

 Those who are familiar with this class of experiments, will 

 have no difficulty in observing the ridge of accumulated fluid 

 remaining after the eye is opened, and gradually falling to 

 its level by the united forces of gravity and capillary attrac- 

 tion. In order to perceive this effect, let the eye be direct- 

 ed to a small point of light, such as the image of a candle 



