ON SENSORIAL VISION. 411 



of its thoughts; while these things obtrude themselves 

 on notice, and, by calling attention to them, direct the 

 train of thought into a channel ,it would not have taken 

 of itself. Retinal impressions they can hardly be, for 

 what is to determine the incidence of pressure or the 

 arrival of vibrations from without upon a geometrically 

 devised pattern on the retinal surface, rather than on its 

 general ground. The effect of some cause in the nature 

 of pressure I on one occasion experienced, and it mani- 

 fested itself quite differently, viz : as an Ocular Spec- 

 trum, consisting in a round, deep purple, feebly lumin- 

 ous spot, dying gradually away into darkness at the 

 borders. It was not exactly in the middle of the visual 

 area, and was caused by no external light : for it was 

 perceived one morning immediately on waking in the 

 morning twilight, and with the face shaded from a direct 

 view of the window. 



(15.) It is quite clear that a regular geometrical pattern 

 cannot be suggested to the imagination by forms having 

 no regularity, however presented to it : so that the 

 explanation which in the other instances adduced might 

 have a certain plausibility, breaks down in these cases. 

 It may be said that the activity of the mind, which in 

 ordinary vision is excited by the stimulus of impressions 

 transmitted along the optic nerve, may in certain cir- 

 cumstances take the initiative, and propagate along the 

 nerve a stimulus, which, being conveyed to the retina, 

 may produce on it an impression analagous to that 

 which it receives from light, only feebler, and which, 

 once produced, propagates by a reflex action the sensa- 



