Contributions to the Physiology of Vision, 107 



III. We come now to some most interesting experiments, 

 viz., those concerning the luminous figures produced by gal- 

 vanism. These experiments were made with a pile of twenty 

 pairs of copper and zinc plates, and layers of cloth dipped in a 

 solution of muriate of ammonia. The pile was constructed 

 in the following manner : zinc, copper, moistened cloth, zinc, 

 copper, &c. ; zinc being the undermost. When the eyes were 

 shut, whilst the positive conductor was placed in the mouth, 

 and the negative wire made to touch the middle of the fore- 

 head, fig. 14 was perceived: it consisted of a dark arch, 

 traversing the centre of the common field of vision, with its 

 concavity upwards, and the extremities losing themselves im- 

 perceptibly in a lateral direction. Above the arch there was a 

 bright violet gleam, the greatest intensity of which was towards 

 the middle of the arch ; laterally from this gleam there were 

 two distinct dark spots, which apparently correspond with the 

 insertions of the optic nerves : the space below the arch was also 

 filled with a bright violet gleam, but so that the greatest inten- 

 sity was seen externally, in the form of luminous roses. When, 

 during the experiment, the right eye only was kept shut, one 

 half only of the figure was seen, but with this difference, that 

 the brightest point of the upper light was seen in the visual 

 axis. When the galvanic poles were changed, the contours of 

 the figure remained the same, but the violet light was changed 

 into a faint yellow glare, the intensities of which were also dis- 

 tributed in an inverse manner, viz., the middle of the field of 

 vision above the arch, and the lateral points below it, being 

 darkest, and the dark points which correspond with the insertions 

 of the optic nerves appearing as distinct bright violet -coloured 

 spots. The direction of the transverse arch was further observed 

 to change in a remarkable manner, according to the different 

 places which the conductors were made to touch during the 

 experiment. When the wire was transferred from the middle 

 of the forehead to the bridge of the nose, the centre of the arch 

 became depressed, and its extremities were raised ; when it was 

 carried along the lower eyelid from the inner towards the outer 

 angle, the arch gradually became indistinct, and ultimately 

 seemed to be divided. At the outer angle the appearance 



