The higher department of fluorescence may with success be employed. Here is something to which believers in the visibility of spirit forms to a camera are quite welcome. At the time, and _àpropos_ of the Mumler trial in New York, I wrote that a good many absurd things have been said _pro_ and _con_ on the subject; but a writer in the latter category, who asserted that anything that is visible to the eye of the camera, and thus capable of being depicted by photography, must therefore necessarily be visible to the human eye, was surely ignorant of that important branch of physics popularly known as fluorescence. Many things are capable of being photographed which to the physical eye are utterly invisible. Why, for that matter, a room (visually dark) may be full of the ultra-violet rays of the spectrum, and a photograph may be taken in that dark light. Objects in a room so lighted would be plainly visible to the lens of the camera—at any rate, they could be reproduced on the sensitive plate, while, at the same time, not an atom of luminousness could be perceived in the room by any person possessing ordinary or normal vision. Hence the photographing of an invisible image, whether it be of a spirit or a lump of matter is not scientifically impossible. If it reflect only the ultra-violet rays of the spectrum, it will be easily photographed, although quite invisible to the sharpest eye. Again, Cromwell F. Varley, F.R.S., well known as one of the most eminent of electricians, says (_Electric_, June, 1871), when passing a current of electricity through a vacuum tube, the results of which were indicated by touches of light about the poles:—“In one instance, although the experiment was carried on in a dark room, this light was so feeble that it could not be seen, and the operators doubted if the current were passing. But photography was at work, and in thirty minutes a very good picture was produced of what had taken place. This,” he says, “is a remarkable fact; indeed, it borders on the wonderful, that a phenomenon invisible to the human eye should have been, so to speak, seen by the photographic lens, and a record thereof kept by chemical agency. It is highly suggestive, and we may anticipate that it will be turned to good account by practical philosophers.”