358 Observations on the Visibility of the Retina ; 



position the foramen centrale was invisible : when it was well 

 seen it presented an appearance as though there were another 

 rim parallel to the one before mentioned, situated a tittle within 

 it on the slope of the cavity ; but its real nature could not 

 be satisfactorily ascertained. By candlelight no trace of this 

 kind could be discovered, excepting at the end of the ellipsis 

 apparently furthest from the nose, where the rim is least de- 

 fined, and a suspicion of some arrangement of this kind may 

 be entertained. 



The base of the optic nerve lies so far from the axis of vi- 

 sion that the writer finds much difficulty in commanding a 

 view of it. By removing the candle, however, very far towards 

 the exterior canthus, a point may be indistinctly seen from 

 which a number of thick blood-vessels radiate in diverging 

 lines towards the remoter parts of the eye, presenting, in con- 

 junction with the more delicate and tortuous ramifications in 

 the centre of the retina, a spectacle of a beautiful and im- 

 posing character. The position of the optic nerve is some- 

 times marked by an undefined luminous white spot, the ex- 

 istence of which may be considered certain, although it has 

 been frequently sought for without success. 



Among the minuter vessels in the centre of the retina, 

 there seem to be many instances of anastomosis, but no distinc- 

 tion can be perceived between veins and arteries in their ap- 

 pearance. 



Such are the facts which the writer has noticed in a fre- 

 quent repetition of the experiment, which succeeded at the 

 first attempt, and has never failed in his hands. Many of 

 them may have been previously observed, or may be thought 

 of little importance; but there is one of unquestionable in- 

 terest, which, as far as his information extends, has never 

 hitherto been noticed, — the shadow in the interior of the fora- 

 men centrale. That it is real shadow, is incontrovertibly evi- 

 dent from mere inspection when the flame is in motion ; but 

 its regular increase in proportion to the diminution of obliquity 

 in the position of the flame, appearing totally at variance with 

 the simplest laws of light and shade, was at first matter of 

 much surprise. The consideration of it has led, however, to 

 the following explanation of the phenomenon, which is here 

 given, not as affording a satisfactory solution of every diffi- 

 culty, but as deserving the attention of those who are far better 

 qualified to decide upon its probability. 



The direction of the shadow and its variations are precisely 

 what they should be, upon the supposition that they depend 

 upon the position and motion of a luminous object situated, 

 not without the eye, but upon the deeply curved surface of 



