_Effect on Plates_ _Refracted by _Visibility_ Lenses_ _Infra-red (heat) rays_ v. slight Yes No _Light rays_ affected Yes Yes _Ultra-violet rays_ strongly affected Yes No _X-rays_ affected No No It appears, then, that ultra-violet rays are suitable for our purpose; infra-red rays, if present in an amount sufficient to affect a photographic plate, would make themselves very evident as heat, and may therefore be ruled out. Ordinary daylight contains ultra-violet rays, as also does the light of the arc lamp and magnesium flash; lamplight, gas-light, and the ordinary electric light, are comparatively deficient in them. But are we to assume that the spirit form is dependent on finding suitable rays in the surrounding ether, or can it produce its own? Perhaps some spiritualist will tell me. This is a point of some practical importance in examining a reputed spirit photograph; for if the spirit is self-luminous its features will be evenly illuminated and without shadows, nor will it cast a shadow on the sitter or background, but rather the reverse. If, on the other hand, the spirit is dependent on the presence of ultra-violet rays from other sources, which it can reflect, then the spirit in the photograph will appear to be illuminated from the same point as the sitter,[9] and by absorption or reflection of the ultra-violet actinic rays which would otherwise have passed on, will cast a shadow on the background. Being a shadow cast by the removal of the ultra-violet rays only, it will of course appear as such in the photograph, but be invisible to the eye.