114 Contributions to the Physiology of Vision. 



pletely disappeared : this is, in fact, a memory peculiar to one 

 particular sense, and thus far, perhaps, the purest instance of 

 subjective vision. The ocular spectrum goes through its regular 

 metamorphoses ; and so far from its distinctness being propor- 

 tionate to the observer's attention, it is most prominent if his 

 look only is fixed to the object his thoughts being otherwise 

 engaged. The ocular spectrum further follows the rotation 

 of the eye, whilst in the mental spectrum the objects maintain 

 their real position, independent of the motions of the eye and 

 the body. Narcotic and spirituous substances, an excited 

 state of mind, some febrile diseases, congestions towards the 

 brain, &c., appear to augment the permanence and clearness of 

 the mental spectra ; that of the ocular spectra is increased 

 during a nervous asthenic state, &c. 



X. Dr. Purkinje says, e I am standing before a white sur- 

 face, and direct my eyes as if I were looking at a very near ob- 

 ject. I perceive in the midst of the field of vision a white 

 transparent circle, with a brownish, semi transparent area, and 

 an indistinct border. If I now discontinue the effort, the 

 brownish area disappears, and the white surface is at its cir- 

 cumference brighter than anywhere else. If, whilst the effort 

 continues, a slight lateral pressure is made on the eyes, the 

 area becomes opaque, of a dark-brown colour, and lined at its 

 outer side with a light violet semitransparent border : the 

 white circle in the midst continues, but on increased pressure a 

 brown central point is seen in it. If the eye is closed, and well 

 secured against external light, the circle in the middle appears 

 dark, and the brown colour of the area is changed into a feeble 

 gleam.' 



During this experiment the brownish area sometimes presents 

 a peculiar phenomenon, , which, according to Dr. Purkinje, is 

 the circulation of the eye becoming visible, in the shape of a 

 series of globules (fig. 23) on each side of the white circle, 

 ascending on the left and descending on the right side. The 

 6 mouches volantes,' Dr. Purkinje is inclined to consider as 

 depending on the same cause : they are best seen if, after violent 

 exertion, the eyes are steadily directed towards a white equally 

 illuminated surface, as the clouded sky, or a snow-field; a 



