194- Mrs. Griffiths on the Spectra of the Eye and Seat of Vision. 



movements, all necessary to the due transmission of impres- 

 sions from without. 



It is not, therefore, that part of the machinery of the eye 

 called the retina which first receives the impressions from 

 without, although it is an expansion of what is called the optic 

 nerve. There is still an agent unknown to us, which, in all 

 cases of the different senses, conveys the impression of external 

 bodies and their qualities to the sensory or consciousness. 

 The retina acts no more in the transmission than what is ef- 

 fected by the pressure of the finger when we wish to inform 

 the mind of the quality of any body. For if the contractile 

 power which conveys the sensation of qualities be not likewise 

 an expansion of the same nerve, originating in the same cen- 

 tral point whence the optic nerve emanates, yet it most as- 

 suredly operates by virtue of the same principle or power 

 which transmits the different sensations that the qualities and 

 appearances of bodies produce. 



Of one thing we are now certain, that the seat of vision lies 

 beyond the machinery of the eyeball ; for by repeated observa- 

 tion and experiment I can see almost every portion of it. 

 This does not arise from any imperfection or obliquity of vi- 

 sion ; for my eyes are, and always have been, in a healthy 

 sound state. I am only sorry that I did not commence the in- 

 vestigation at an earlier period. It remains still a matter of 

 doubt with me, whether an eye younger than forty can be 

 sensible to the appearances which those beyond that age are 

 capable of comprehending. I do not say that the eye must be 

 injured by disease, for old age is not a disease, but I suspect 

 that an eye in full vigour has all its parts so nicely adjusted, 

 and the elasticity is so on the qui vive, that there is an inti- 

 mate adhesion and blending of the whole machinery, no one 

 portion of which is more relaxed than the other. 



Nine or ten years ago I discovered that, by placing the light 

 of a candle at a certain distance from the eye, and allowing it 

 to fall obliquely— as I stated in p. 306. of Our Neighbourhood* 

 — on the cornea, I could see part of the interior of my own eye. 

 I recollect drawing a map of the spectrum as it invariably 

 appeared, and several years after I showed it to Dr. Hosack. 

 Since that period I can bring the spectrum to my mind either 

 with the eye half closed or nearly open, either by means of a 

 candle, or from a ray of sun-light. I subjoin a map of the 

 spectrum as it appears in both eyes, for, as happens to Mr. 

 Horner, each eye presents a somewhat different appearance. 



The spots in the margin are dark in the centre, with a halo 

 of paler light around them. The golden appearance of the 



* The title of a Novel by Mrs. Griffiths. 



