All the photographs were obtained by the general method of double exposure and substitution, the substitution being effected at a different point on each occasion; the methods used, or slight variations of them, are all described in the section on “Methods of Fraud.” Now I maintain that the conditions imposed upon me were as strict, or stricter, than any professional medium allows. If an amateur photographer but little practised in sleight-of-hand can under such conditions deceive intelligent observers--not once, but several times over--how much easier will it not be for the professional spirit photographer, who makes such frauds his business? _C.--Internal Evidence of Fraud_ Since spiritualists claim that the presence of invisible spirits may be detected by photography, it seems reasonable to inquire how far this is compatible with established physical facts. If a plate is wrapped in paper and submitted to “spirit influences”--whatever these may be--never being exposed in a camera at all, and on development shows faces or writing, I personally can only find one explanation--trickery. But if a plate is duly exposed with camera and lens, and unseen faces appear on development, the matter is not quite so simple. For it is well recognised that the camera may record what is invisible to the eye; invisible stars are detected by the photographic plate, and anyone who has examined a nebula or comet through a telescope, after seeing a photograph of the same object, realises this fact to his disappointment. Similarly a can of hot water may be photographed, by a long exposure, in a perfectly dark room; and another well-known instance of a similar phenomenon is Sir Robert Ball’s story of photographing some writing on the side of the “Great Eastern,” years after it had been painted out and rendered invisible. Light, as is well known, is now regarded as consisting of waves in the ether. Ether waves are known to exist over a very large range of wave-lengths; some are comparatively long waves, some are short. The properties of these waves depend upon their wave-length; those visible to our eyes, which we call “light rays,” form only a small section of the complete scale; comparing them with sound waves they correspond to approximately one octave of the whole musical scale. Ether waves of greater or lesser wave-length than light, _i.e._, of lower or higher octaves, have very different properties. Radiant heat and ultra-violet rays are the ether waves nearest in wave-length and properties to light; X-rays and the waves responsible for wireless telegraphy appear to be similar waves further removed along the scale of wave-length. Now in order to photograph an invisible object we require rays that (_a_) affect a photographic plate; (_b_) are capable of refraction by a lens; and (_c_) are invisible to the eye. The properties of the principal known rays concerned may be summarised as follows: