with Special Reference to the Vision of Insects. 321 



image formed by compound eyes is erect, while that formed 

 by single eyes, such as ours, is inverted. Neither position, 

 however, nor a sideward position, nor any other, would 

 be incompatible with our seeing the objects of the world 

 around us exactly as we now do. For the direct physical 

 adjunct of a visual perception in our mind of a point of 

 the object, is not any event in the eye or along the optic 

 nerve but in a more deep-seated part of the brain, probably 

 in its occipital lobes which lie in the back of the head, over 

 the cerebellum. Now (speaking from the physical stand- 

 point) the way in which this event in the occipital lobe is 

 usually evoked is by light from the point of the object being- 

 guided through the eye to one of the rods or cones, after 

 which some event travels along one of those nervelets with 

 attendant nerve-cells which penetrate the retinal layer from 

 the expansion of the optic nerve, and each of which is asso- 

 ciated with one individual rod or cone. This is succeeded 

 by some event along one fibril of the optic nerve, after, which 

 there seem to follow other events within the brain, which 

 finally lead up to that particular event which, and which 

 alone, is the true physical adjunct of the visual perception in 

 our mind — our perception of that point of the object from 

 which the light set out to enter the eye. I, for convenience, 

 speak of this event as situated in the occipital lobe, although 

 its location can hardly be said to be ascertained. 



Now it is evident that the image on the retina is only one 

 link in this long chain of physical causes and effects, and 

 that the image might be erect as it is in the compound eyes 

 of insects, or inverted as in our eyes, or might have any other 

 orientation, and that nevertheless the positions of the rod or 

 cone, nervelet, fibril of optic nerve, &c, could be so disposed 

 as to produce precisely the same final event within the occipital 

 lobe of the brain, as now occurs. Now it is this last alone 

 which is essential, the others being only instrumental in 

 bringing it about : it alone is the true physical adjunct of 

 the visual perception which becomes part of the mind. 



Again, although the train of causes and effects described 

 above is the usual process by which this adjunct of percep- 

 tion is evoked, it is not by any means the only way in which 

 it can be brought about, as is conspicuously manifested by 

 dreams, and may be detected by a careful introspective study 

 of the memory of visual perceptions. I am of opinion that 

 in all cases, when remembering a past scene, there is some dim, 

 usually a very dim, recurrence of the perception, or of parts 

 of it : at all events, under some circumstances, this is dis- 

 tinctly the case. When, unfortunately, we lie awake for 



