from The Century Dictionary.
- Pertaining to the retina exclusively: as, idioretinal light, a subjective sensation of light originating in the retina itself.
Sorry, no etymologies found.
The greater ease of vertical as compared with horizontal movements recalls an observation of Ladd,  in which the idioretinal light was willed into the shape of a cross.
If at any time, except when the eyes are quite fresh, one closes one's eyes and attends to the after-images, some will be found which are so faint as to be just barely distinguishable from the idioretinal light.