Contributions to the Physiology of Vision. 105 



pressure be exerted on the middle of the eye, a large luminous 

 ring is seen, which, on close attention, will be found to resemble 

 fig. 5. It consists of numerous small oblong rectangles, ob- 

 liquely arranged, and more or less bright and obscure ; the 

 sides of the figure have an oblique position, and its form is 

 that of a rhombus with obtuse angles ; the centre, as well as 

 the space round the rhombus, is dark, but gradually becomes 

 traversed by a luminous star (fig. 6). The rectangles become 

 more intensely illuminated, and after some time one of the 

 angles is filled by a yellowish- white spot with distinct edges 

 (fig. 7), which progressively enlarges, and ultimately occupies 

 the entire rhombus. In this luminous space, which is now of 

 a bluish colour, very small circular lines are observed, which 

 are either concentric or variously intersect each other, and 

 seem to be in a continual glimmering fluctuation (fig. 8) : at 

 the circumference of the rhombus there is a very narrow 

 orange-coloured ring, and round this is occasionally seen 

 beyond a dark interstice a larger ring of the same colour. On 

 discontinuing the pressure, the figure successively repasses 

 through all the metamorphoses in an inverse manner to that in 

 which they have taken place. 



When a strong pressure is made on the eye, the figure 9 is 

 seen. The serpentine rays seem to proceed from the centre, 

 and are in continual fluctuation from brightness to obscurity ; 

 after some time the black intervals between them become filled 

 with squares (fig. 10) ; the radiated figure then gradually dis- 

 appears, and the square field itself terminates at last in the 

 luminous rhombus. The pressure being still increased, the 

 appearance represented in fig. 11 is perceived ; the luminous 

 spots alternately appear and disappear, and during their dis- 

 appearance are replaced by black spots, which again give place 

 to the luminous ones ; the larger spots, which are of a bluish 

 tint, appear and disappear more slowly than the smaller ones, 

 which occur nearer the centre. On continuing the pressure, 

 the small luminous spots gradually fade away, while the larger 

 spots near the circumference remain much longer, but they 

 ultimately also disappear in succession : in the mean while a 

 vague and continually fluctuating gleam has been dawning, 

 which now develops itself into various groups of spots, rings, 



