Some very striking phenomena in photographing the invisible may be produced by the agency of fluorescence. Figures depicted upon a background by one or other of certain substances I shall presently name, although invisible to the eye, may become visible to the camera. Of these, the best known, although not the most effective, is disulphate of quinine. Such a solution, although to the eye it is colourless like water, is to the camera as black as ink. Fill three phials respectively with water, quinine, and common writing ink, and you have two whites and one black; but photograph them, and you have two blacks and one white. The camera has reduced the transparent quinine solution to the colour of the ink. Those of you who may care to experiment in this direction, please take notice that the quinine must be acidulated with sulphuric acid, and that hydrochloric acid, even a small trace, will destroy this property. Among other substances that are fluorescent, or that change the refrangibility of rays of light, are mineral uranite, certain salts of uranium, canary glass, alcoholic solution of chlorophyll, æsculine, tincture of stramonium seeds, and of turmeric. There are others known to be still better, but my experiments in this direction are yet too incomplete to warrant my even indicating them. Let me for a moment enter the realm of speculation, and assume that there are really spirits invisible to the eye but visible to the camera and to certain persons called seers or clairvoyants only. Might we not suggest that there is some fluorescent compound in the eyes of such persons not present in those whose eyes are normal, and that it is to this they owe their seeing powers? Some of you may probably be aware that Dr. Bence Jones and other philosophers have actually established the fact of such fluorescent substances being found in some eyes. May this throw any light upon the recognised fact of certain animals being able to see in the dark? When the subject of fluorescence is more thoroughly investigated (it is a discovery of Sir D. Brewster, who was followed by Herschel and Professor Stokes, and is as yet but of yesterday), we may hope for a vast accession to our knowledge of subjects as yet very slightly understood. At the Bradford meeting of the British Association for the Advancement of Science, in 1873, Dr. Gladstone, F.R.S., demonstrated before the Mathematical and Physical Section what I have said respecting invisible drawings on white cards having produced bold and clear photographs when no eye could see the drawings themselves, and I brought away back to London these photographs, and, for aught I know, may have them still. To prevent this disquisition from being too dry, I will here introduce a fanciful sketch I wrote _àpropos_ of Dr. Gladstone’s demonstration at the time mentioned:—