402 Proceedings of the Royal Institution, 



yellow, which relieves the eye from the duty of compensating the 

 impression. The same applies to any two separate primitives. 



Dr. Darwin was of opinion that mere fatigue of the organ, by any 

 one colour, excited re-action, and produced the spectrum of another. 

 Tliis explains nothing ; and though {t?%ay be difficult to find a struc- 

 ture for this curious function in the eye, there must be a physical 

 and positive power possessed by some organ in the eye, which, sti- 

 mulated by the coloured ray that falls upon it, is so exquisitely 

 contrived, that, though whilst excited by a coloured ray, it imparts 

 to the sensorium the idea of that colour, red for instance, yet it 

 excites in the organ itself that idea of colour which subdues the 

 excitement, when the impinging colour is removed, and restores to 

 the organ of vision that perfect condition, which is requisite for the 

 reception of new impressions. 



Of some of the structures in the eye the use and principles are per- 

 fectly understood ; and the laws which govern the refraction of lenses, 

 in instruments made to assist the eye, govern also the crystalline 

 lens of the eye itself. The profound investigations of those laws of 

 optics by mathematics have been among its most beautiful applica- 

 tions ; but the researches of physiologists have not been at all com- 

 mensurate. They have been content with form, and regardless of 

 function ; and when a new structure has been first observed, the dis- 

 coverer has been content with the honour of giving his name to a 

 membrane or a duct, and left others to find out their use ; the extreme 

 delicacy of the various parts of the organs of vision renders it a sub- 

 ject of great difficulty ; but Mr. Brockedon thought that it had not 

 been so fairly grappled with as other subjects of science, which did 

 not present half the beauty or interest in the investigation. He ven^ 

 tur>d to offer, only as conjecture, to be examined by those who had 

 more knowledge, skill and opportunity than himself, that, in some parts 

 of the eye might be found an organ with a structure analogous to the 

 striae, which, when parallel and equidistant, and amounting to 2000 

 or 3000 in an inch, were known, in reflecting, to disperse the light 

 into colours of qualities, peculiar to the approximation of the striae. 

 Such a structure is certainly found in the delicate membrane disco- 

 vered some years ago by Dr. Jacob of Dublin. When powerfully 

 magnified, as drawn by Mr. Bauer of Kew, in the Phil. Trans, of 



