Contributions to the Physiology of Vision, 103 



unless the experiment be continued for some length of time, 

 Figs, 1, 2, 3 and 4 represent the phenomena as observed by 

 Dr. Purkinje in his right eye. 



Fig. 1 consists of small squares, chequered as in a chess 

 board, and alternately bright and dark ; this entire figure is 

 bounded by zigzag lines, which arc continually varying in 

 direction, length, and brightness, and which appear rather more 

 illuminated than the squares ; at the centre of the square field 

 is a dark point, with a luminous area, surrounded by rapidly 

 moving semicircular lines, which nearly resemble rose-leaves in 

 form ; these are around and principally below the luminous 

 area ; below these semicircular lines is a field of hexagons, the 

 circumferences of which are gray, and the centres white. This 

 figure, Dr. Purkinje says, may be obtained very distinctly, 

 and without any admixture of the other figures, if the experi- 

 ment be modified so that the eye, being open, is directed to- 

 wards an equally illuminated white wall, and the spread fingers 

 are moved before it ; if the experiment be made as before de- 

 scribed, the secondary figures rather predominate. The ap- 

 pearances also take place under various other circumstances 

 and modifications ; for instance, the semicircular lines at the 

 centre of the figure are particularly visible when the eyes are 

 directed as near as possible to the flickering flame of a candle. 

 The square field is also seen by looking at Newton's circle of 

 colours, when it is in rapid motion ; in this case it is not neces- 

 sary that the colours be distributed in any particular manner, 

 for the experiments will succeed, if the segments of the disc be 

 merely alternately light and dark ; the nearer the segments are 

 to each other, the less rapid the motion of the circle is required 

 to be, but bright sunshine is indispensable to the experiment. 

 Lastly, the figure is seen when a wheel rapidly revolves be- 

 tween the eye and the sun, or a strong light ; and it appears 

 accordingly that the general condition of the phenomenon is a 

 rapid alternation from light to shade. 



The secondary figures, as Dr. Purkinje calls them, are in- 

 distinct when the experiment is made whilst the eyes are open ; 

 they appear under two modifications, the rectangular spiral, 

 and the star with eight rays : at the commencement of the 

 experiment, whilst the eyes are not over excited, both figures 



