a dry piece of wood has a very definite smell, there is something given off which can affect our nose and there is no reason why it should not change, by purely chemical action, the photographic plate. This action of wood on the plate is prevented by interposing a sheet of glass. Frankland (1898) has described similar vapors coming from colonies of _Bacillus proteus vulgaris_ and _B. coli communis_ which affect a photographic plate laid directly over the colonies in an open petri dish. There is no effect if the glass cover of the petri dish is between plate and bacteria. There is, then, no specific emission of X-rays or similar penetrating radiation from luminous tissues which will affect the photographic plate through opaque screens. A similar conclusion is reached if we attack the problem in another way. X-rays and radium rays (Becquerel rays) cause fluorescence of ZnS, barium platinocyanide, willemite (Zn_{2}SiO_{4}), and calcium tungstate. Coblentz (1912) showed that the firefly will cause no fluorescence of a barium platinocyanide screen and I have been unable to detect fluorescence of zinc sulphide, barium platinocyanide, zinc silicate (willemite) or calcium tungstate shielded from _Cypridina_ light by black paper, although the light of this organism is quite bright enough to cause phosphorescence of zinc sulphide without the black paper. The samples of the above four substances all showed fluorescence in presence of radium rays, but only the ZnS phosphoresces after exposure to light rays, although the willemite was phosphorescent after exposure to the ultra-violet. While photometry at low intensities is a difficult procedure at best, if the light varies in intensity or is a flash, accurate measurements become well-nigh impossible. The figures given for intensity of animal luminescence must, therefore, be accepted with a realization of the difficulties of measurement. By candle is meant the international candle, unless otherwise specified, equal to 1.11 Hefner candles (H. K.) 0.1 pentane lamp and 0.104 carcel units. It is a measure of intensity. Amount of light, or light flux, measured in lumens, is that emitted in a unit solid angle (area/_r_^2) by a point source of one candle-power. One candle-power emits 4π lumens. The latest figure for the mechanical equivalent of light at λ = .566 is .0015 watt (Hyde, Forsyth and Cady, 1919), _i.e._, 1 lumen = .0015 watt. One watt is 10^7 ergs (one joule) per second. The illumination (of a surface) is that given by one candle at one metre, the candle metre (C.M.) or lux. The surface then receives one lumen per square metre. A metre kerze (M.K.) is the illumination given by one Hefner candle at one metre distance.