Yet I believe that we have still persistent in our brain-structure some dim vestige of the transition from that early undifferentiated continuous sensitivity to our existing specialisation of sense. Probably in all of us, though in some men much more distinctly than in others, there exist certain _synæsthesiæ_ or concomitances of sense-impression, which are at any rate not dependent on any recognisable link of association.[101] My present point is that such synæsthesiæ stand on the dividing line between percepts externally and internally originated. These irradiations of sensitivity, sometimes apparently congenital, cannot, on the one hand, be called a purely mental phenomenon. Nor again can they be definitely classed under external vision; since they do sometimes follow upon a mental process of association. It seems safer to term them _entencephalic_, on the analogy of _entoptic_, since they seem to be due to something in brain-structure, much as entoptic percepts are due to something in the structure of the eye. I will, then, start with the synæsthesiæ as the most generalised form of inward perception, and will pass on to other classes which approach more nearly to ordinary external vision. From these entencephalic photisms we seem to proceed by an easy transition to the most inward form of unmistakable entoptic vision--which is therefore the most inward form of all external vision--the flash of light consequent on electrisation of the optic nerve. Next on our outward road we may place the phosphenes caused by pressure on the optic nerve or irritation of the retina. Next Purkinje's figures, or shadows cast by the blood-vessels of the middle layer upon the bacillary layer of the retina. Then _muscæ volitantes_, or shadows cast by motes in the vitreous humour upon the fibrous layer of the retina. Midway, again, between entoptic and ordinary external vision we may place _after-images_; which, although themselves perceptible with shut eyes, presuppose a previous retinal stimulation from without;--forming, in fact, the entoptic sequelæ of ordinary external vision. Next comes our ordinary vision of the external world--and this, again, is pushed to its highest degree of externality by the employment of artificial aids to sight. He who gazes through a telescope at the stars has mechanically improved his end-organs to the furthest point now possible to man.