PHYSICAL NATURE OF ANIMAL LIGHT 63 



■willemite (Zn2Si04), and calcium tungstate. Coblentz 

 (1912) showed that the firejBy will cause no fluorescence of 

 a barium platinocyanide screen and I have been unable 

 to detect fluorescence of zinc sulphide, barium platinocya- 

 nide, zinc silicate (willemite) or calcium tungstate shielded 

 from Cypridina light by black paper, although the light 

 of this organism is quite bright enough tojcause phospho- 

 rescence of zinc sulphide without the black paper. The 

 samples of the above four substances all showed fluores- 

 cence in presence of radium rays, but only the ZnS phos- 

 phoresces after exposure to light rays, although 

 the willemite was phosphorescent after exposure to 

 the ultra-violet. 



While photometry at low intensities is a difficult pro- 

 cedure 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 iatensity. 



Amount of light, or light flux, measured in lumens, is 

 that emitted in a unit solid angle (area/r*) by a point 

 source of one candle-power. One candle-power emits 4ir 

 lumens. The latest figure for the mechanical equivalent 

 of light at A = .566 is .0015 watt (Hyde, Forsyth and Cady, 

 1919), i.e., 1 lumen = .0015 watt. One watt is 10^ ergs (one 

 joule) per second. 



The illumination (of a surface) is that given by one 

 candle at one metre, the candle metre (CM!) or lux. The 



