104 Contributions to the Physiology of Vision. 



appear, as it were, mingling with each other, the radiated 

 figure evidently predominating (fig. 2) ; but as the experiment 

 continues, the rectangular spiral (fig. 3) becomes more visible, 

 and the star gradually disappears : the central line of the 

 spiral is the smallest and darkest, as will be seen by the figure, 

 and has an oblique direction to the right and below ; the line 

 itself consists of a darker axis and a bright margin, and is 

 divided, as it were, into joints ; towards the periphery of the 

 figure the axis becomes enlarged, and fades to a greyish tint ; 

 the lateral margin also loses its brightness, and at the termina- 

 tion of the spiral line the illumination seems even to be inverted 

 that is, the centre appears light, and the circumference dark : 

 it is, however, impossible to determine with great accuracy the 

 external parts of the figure ; the intervals between the coils of 

 the spiral are occupied by a faint gleam of the squares of fig. 1. 



In the two oblique lines of the star (fig. 4), the light axes 

 are brighter than in the other two lines ; in the latter, on the 

 contrary, the dark borders are of a deeper black. The spiral 

 and radiated figures are in continual motion and fluctuation : 

 sometimes the rectangular spiral changes into a triangular one ; 

 at other times the centre of the star dissolves, and the rays in- 

 tersect each other at various points, or become parallel, or form 

 squares, triangles, &c. The four figures above described are, 

 however, those which most frequently occur ; and though, as 

 Dr. Purkinje judiciously remarks, these subjective phenomena 

 might appear to other eyes different from what he observed, 

 yet the experiments made by others at his request seem to 

 confirm his own observations : we may therefore, perhaps, be 

 justified in concluding that the above phenomena do not depend 

 on a morbid or individual condition, but physiologically result 

 from the very organization of the human eye. 



The figures in Dr. Purkinje's left eye, the sight of which 

 was very weak, were very indistinct, but did not in any other 

 respect appear different from those perceived by the right eye : 

 the squares were more like network formed by curved lines ; 

 the secondary figures were apparently the same as before, but, 

 as might be supposed, were in the opposite direction. 



II. Fujures produced by pressure on the eyebalL~If gentle 



