Contributions to the Physiology of Vision. 109 



wards, and, on approaching towards the middle, its lateral 

 ends bend until they unite and form a luminous ring, which is 

 then dissolved into darkness, as in the preceding case. 



3. The luminous streak comes from below and moves up- 

 wards. Sometimes the streaks move in rather an oblique 

 direction. 



4. The appearance is as in fig. 18, and the nebulous streak 

 moves in a circular direction, like the sails of a windmill. 



When the experiment has been continued some time, and 

 the attention becomes exhausted, all regular appearances 

 dissolve themselves into a fluctuation between light and obscu- 

 rity, which ultimately terminates in a feeble gleam covered as 

 it were by a veil. 



V. The following is another instance of subjective vision. 

 When the eyes are fixed on a large illuminated surface (a 

 white wall, a regularly clouded sky, &c.), the observer sees, 

 after a few seconds, bright points suddenly starting up in the 

 midst of the field of vision ; they rapidly disappear, making 

 room for black spots, which also quickly dissolve. If, after 

 the appearance of the bright points, the eyes are shut or 

 directed to a dark surface, the phenomenon continues ; but in 

 a milder light the bright appearances are changed into a feeble 

 glimmer. The bright points are also seen when the eyes are 

 shut before they have appeared. 



VI. Place of insertion of the optic nerve. It was first 

 shewn experimentally by Mariotte, and was afterwards mathe- 

 matically ascertained by Euler and Bernoulli, that the image 

 of an object disappears in that point of the field of vision which 

 corresponds with the insertion of the optic nerve. Besides 

 this, there are some other circumstances under which objects 

 within the field of vision will disappear. If a number of black 

 dots are made on an equally illuminated surface, with one of 

 them in the middle, and the eye is fixed to the central dot, an 

 indistinct nebulous floating begins, and some of the dots, some- 

 times all of them, alternately disappear and reappear, whilst 

 the light ground on which they are marked remains unaltered. 



