110 Contributions to the Physiology of Vision. 



That the place of insertion of the optic nerve is not 

 entirely insensible to light, as has been sometimes stated, 

 appears from the following simple experiment : If a small 

 flame be placed in the projection of that part of the field of 

 vision which corresponds to the insertion of the optic nerve, it 

 will directly disappear, but in its stead a beautiful red nimbus 

 is seen; if the flame is slightly moved in a lateral direction, 

 upwards, or downwards, there appears in the opposite side of 

 the nimbus a dark gap, which spreads parabolically down- 

 wards or upwards, and the margin of which is coloured 

 with the light of the flame. If the flame is moved in a small 

 circle, the shade also shews a circular motion, being always 

 opposite to that of the flame. 



VII. If the eye, being well covered, is quickly turned out- 

 wards, a large luminous ring (fig. 19) will be seen, the light 

 of which is in a constant glimmering fluctuation : this pheno- 

 menon is particularly striking in the morning, immediately 

 after awaking, and then, besides the luminous orb, the entire 

 field of vision, but particularly the upper and lower parts of it, 

 is filled with large equidistant sparks. The central area of 

 the orb appears of a grey colour if, during the experiment, the 

 eyes are open and directed towards a white surface ; and of a 

 deep-blue colour if they are shut and directed towards the sun* 

 If, during the experiment, the eye is directed to any other 

 colour, the inner surface of the ring is not of the complemen- 

 tary, but of the same colour, though rather deeper. Round 

 the luminous orb, towards the centre of the field of vision, 

 there are concentric bright streaks with dark intervals (fig. 20). 

 It appears that the luminous orb is produced by the nerve 

 being forcibly stretched by the rapid lateral motions of the eye. 



VI II. Another very interesting experiment is the following. 

 If a flame, at about two or three inches distance, is slowly 

 moved before the right eye in various directions, figure 21 

 appears painted as it were in the luminous area round the 

 flame. The vessels, for such they evidently appear to be, seem 

 to proceed from the insertion of the optic nerve, and consist 



