116 Contributions to the Physiology of Vision. 



concentric white and black lines are seen (fig. 25), similar to 

 the appearance of fig. 20 ; they extend over the spot in the 

 middle of the visual field (fig. 20), and are always parallel 

 wherever the pressure may be exerted. On the paper there 

 appears, at the same time, a large black circular spot, the centre 

 of which is of a dark bluish green, or deep violet, similar to the 

 eye of a peacock's tail, sometimes with fragments of the vas- 

 cular figure (fig. 18). That side of the spot which is directed 

 towards the middle of the field of vision, touches at the above- 

 mentioned parallel lines ; the opposite side is bordered by a 

 yellowish-white gleam, which, on increased pressure, reaches 

 as far as the middle of the black spot. 



4. If by placing the piece of wood between the orbit and 

 eye-lid, the pressure is exerted on the posterior point of the 

 globe, the parallel lines are seen extending towards the middle 

 of the field of vision, and terminating in a white semilunar 

 streak, which, in its concavity, contains a small bright circle, 

 and at the convex portion of which there is a brownish semi- 

 lunar spot ; both spots follow all the motions of the coloured 

 eye, and turn round the centre of the field of vision as on an 

 axis. If the pressure is increased, the coloured eye advances 

 towards the white semilunar streak, so as to cover it entirely 

 with the exterior of its middle portion, which remains as a white 

 circular spot in the middle; the brown semilunar spot also 

 disappears. 



5. If the pressure on the globe is suddenly discontinued, the 

 white circular spot as suddenly moves outwards, and in its 

 stead a light-brown violet cloud remains, which is divided by a 

 white streak into two parts, the upper of which is larger and 

 darker ; this cloud, especially the middle portion of it, gene- 

 rally remains for a considerable time, and greatly impedes clear 

 vision. 



6. The experiment in question shews also the coincidence 

 of the two fields of vision ; for if each eye is pressed at the cor- 

 responding place, the luminous rings are always seen to coin- 

 cide. 



7. If the eye is well covered during the experiment, the 

 colours in the middle of the circular spot, as well as the margin 

 round the periphery, are luminous ; the concentric lines are very 



