106 Contributions to the Physiology of Vision. 



and squares, which after some time arrange themselves into 

 small squares and larger hexagons (fig. 12). To see this 

 figure distinctly the pressure must be equal, and the eye be 

 kept steady, for on the least motion of the eyeball a general 

 fluctuation prevails, and no defined figure can be distinguished : 

 sometimes curved lines are seen, which rapidly move round a 

 centre in alternate directions. If the pressure be discontinued, 

 luminous ramificatibns, the fragments of a figure which will be 

 hereafter described, appear (fig. 13) : the appearance subse- 

 quently terminates in the luminous rhombus, &c. 



When the luminous rhombus has been produced by gentle 

 pressure, if the eye be opened and directed towards the un- 

 clouded sky, numerous parallel and converging grey semi- 

 transparent lines will be perceived, which evidently correspond 

 to the bright oblong rectangles, and on closing the eye they 

 again become luminous. On opening the eye during the 

 appearance of figs. 9 and 10, the daylight is at the first 

 moment invisible : suddenly the figure bursts, as it were, at 

 the centre, rapidly opens towards the circumference, and at 

 last entirely disappears. If the eye be opened when fig. 12 

 has been produced by strong pressure, twenty seconds some- 

 times elapse before the daylight can be seen, and even then it 

 is obscured for a considerable time by opaque lines and spots. 



Similar figures to those produced from pressure, particularly 

 the rectangles, are produced by impeded circulation through 

 the brain, violent exertions, and the use of narcotics ; they 

 appear also during fainting fits, strong mental emotions, &c. 



Dr. Purkinje institutes a comparison between the above and 

 acoustic figures, and concludes that both phenomena are objec* 

 tively identical : the primary rectangular, &c., figures he con- 

 siders to be analogous to the small reticulated undulations 

 communicated from a sounding plate to the surface of a liquid ; 

 the secondary ones to those which are caused by the intersection 

 of the undulations. He does not, however, state where he con- 

 ceives this undulatory motion to be : probably they arise in the 

 humours of the eye, and are thence communicated to the retina ; 

 but he speaks only of a contraction of the eyeball as its imme- 

 diate cause. The luminous rhombus he considers to be caused 

 by the lens ; the figure 13 by the central vessels of the eye, &c. 



