Contributions to the Physiology of Vision. 115 



large quantity of bright points (fig. 24) are then seen, which, 

 like shooting stars, suddenly arise and disappear after a rapid 

 motion, in various straight and curved lines. On close atten- 

 tion, it will be found that every light point is accompanied by 

 a shade at the opposite side of the field of vision, and that 

 also between the small, larger but less bright points are slowly 

 moving. These larger points are very distinctly seen after 

 violent exertion, particularly after lifting a weight : they move 

 from the extreme margin of the field of vision towards the mid- 

 dle, and are in a straight or bent direction, always accompanied 

 by a shade at the opposite side; the nearer they come to the 

 middle, the less distinct and shining do they appear, and the 

 less dark are their 'shadows. As they are visible only as long 

 as the eyes are held open, and as they require a strong and 

 equal light, in order to be seen, they must be considered as 

 differing from the bright points described above, as far as these 

 evidently depend on the different state of the various points of 

 the retina, whilst the phenomenon in question is caused by 

 external bodies, with reference to the retina, viz., according to 

 Dr. Purkinje's opinion, by free blood-globules in the aqueous 

 humour; which, according to their different distances from the 

 crystalline lens, are seen of different size and distinctness. 



XI. Luminous Rings. This phenomenon, which is suf- 

 ficiently known to be caused by lateral pressure on the eye, 

 has been carefully examined by Dr. Purkinje : the following 

 are the results of his experiments : 



1. If the observer makes an effort, as if to look at something 

 very near, the slightest pressure produces the luminous ring ; 

 whilst, on looking at a distance, the pressure must be consider- 

 ably increased. 



2. The rings, as well as the places of insertion of the optic 

 nerve's, are most vivid in the morning, and the proximate cause 

 of both phenomena appears to be identical, viz. ; pressure on 

 the retina. 



3. If a piece of white paper is held in the inner angle, and 

 whilst the eye is as much as possible directed towards it, the 

 observer presses, with a small conical piece of wood, on the 

 external side of the globe, near the orbit, a great number of 



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