It will be convenient for future reference if I draw out this parallel somewhat more fully. I compare, then, man's gradual progress in self-knowledge to his gradual decipherment of the nature and meaning of the sunshine which reaches him as light and heat indiscernibly intermingled. So also Life and Consciousness--the sense of a world within him and a world without--come to the child indiscernibly intermingled in a pervading glow. Optical analysis splits up the white ray into the various coloured rays which compose it. Philosophical analysis in like manner splits up the vague consciousness of the child into many faculties;--into the various external senses, the various modes of thought within. This has been the task of descriptive and introspective psychology. Experimental psychology is adding a further refinement. In the sun's spectrum, and in stellar spectra, are many dark lines or bands, due to the absorption of certain rays by certain vapours in the atmosphere of sun or stars or earth. And similarly in the range of spectrum of our own sensation and faculty there are many inequalities--permanent and temporary--of brightness and definition. Our mental atmosphere is clouded by vapours and illumined by fires, and is clouded and illumined differently at different times. The psychologist who observes, say, how his reaction-times are modified by alcohol is like the physicist who observes what lines are darkened by the interposition of a special gas. Our knowledge of our conscious spectrum is thus becoming continually more accurate and detailed. But turning back once more to the physical side of our simile, we observe that our knowledge of the visible solar spectrum, however minute, is but an introduction to the knowledge which we hope ultimately to attain of the sun's rays. The limits of our spectrum do not inhere in the sun that shines, but in the eye that marks his shining. Beyond each end of that prismatic ribbon are ether-waves of which our retina takes no cognisance. Beyond the red end come waves whose potency we still recognise, but as heat and not as light. Beyond the violet end are waves still more mysterious; whose very existence man for ages never suspected, and whose ultimate potencies are still but obscurely known. Even thus, I venture to affirm, beyond each end of our conscious spectrum extends a range of faculty and perception, exceeding the known range, but as yet indistinctly guessed. The artifices of the modern physicist have extended far in each direction the visible spectrum known to Newton. It is for the modern psychologist to discover artifices which may extend in each direction the conscious spectrum as known to Plato or to Kant. The phenomena cited in this work carry us, one may say, as far onwards as fluorescence carries us beyond the violet end. The "X rays" of the psychical spectrum remain for a later age to discover.