548 SCIENCE PROGRESS 



illumination naturally leads us to consider the allied phenomenon 

 of phosphorescence. By this term is generally understood the 

 transformation of light from one frequency to another which 

 persists after the exciting stimulus is withdrawn. 



Inventors have long cherished the fascinating idea of a means 

 of securing a cold phosphorescent light, portable and self- 

 contained, which should absorb light by day and return it 

 automatically when darkness had fallen. Fanciful such a sugges- 

 tion may well appear at present, yet it might conceivably be 

 realised in the future as a result of the persistent patient research 

 on phosphorescence and fluorescence which is now going on in 

 various parts of the world. At present, there are obvious diffi- 

 culties. Phosphorescent materials, even under the most favour- 

 able circumstances, yield but a feeble light of short duration. 

 Moreover the colour of the light yielded by most phosphorescent 

 substances, generally a spectral blue or green, is hardly suitable 

 for practical illumination. 



Yet within recent years appreciable progress has been made- 

 Our stock of phosphorescent materials has been considerably 

 enlarged. We understand better how to prepare such substances 

 in a highly active state and we have at our command a much 

 greater range of colour. Moreover varieties of insects are 

 known which give light of almost every conceivable colour and 

 sometimes of a surprising intensity and the remarkable effect of 

 phosphorescent organisms on some of the Canadian lakes has 

 often been remarked. May we not, therefore, hope ultimately to 

 produce a culture of bacteria gifted with special light-giving 

 qualities ? And may not chemists eventually succeed in pro- 

 ducing synthetically and in large quantities the substances which 

 yield the light in the case of these phosphorescent insects ? 



Another respect in which progress has been made is in the 

 means of exciting phosphorescence. The new mercury lamps 

 having tubes of special quartz or uviol glass are exceptionally 

 powerful producers of ultra-violet rays, on the exciting action of 

 which the phenomena of fluorescence and phosphorescence 

 largely depend. By the aid of such lamps wonderful effects are 

 said to have been produced in America, although little has yet 

 been published on the subject. The writer was recently given 

 to understand that its success for the production of novel stage 

 effects has already been proved. Mechanical butterflies coated 

 with luminous paint are excited and then set in motion and by 



