242 THE ORIGIN OF LIFE 



no means clear, however, that the phenomenon we 

 are now dealing with is really a case of thermo- 

 luminescence, in so far as the effect is said to be 

 produced only under the combined action of the 

 exciting more refrangible, and the destructive less 

 refrangible ones, and not by the former alone. 

 Stokes in his paper (Proc. Roy. Soc, 1882, p. 65) 

 emphasises this point. He found that the colour 

 of the phosphorescence changes under the influence 

 of the less refrangible rays, and in his opinion the 

 destructive effect is due to the combined influence 

 of both. He describes an interesting experiment 

 upon this point : "a phosphorescent tablet was laid 

 horizontally on a lawn on a bright day towards 

 evening when the sun was moderately low, and 

 a pole was struck in the grass in front of it, so 

 as to cast a shadow on the tablet. After a brief 

 exposure the tablet was covered with a dark cloth 

 and carried into a dark room for examination. 



" It was found that the place of the shadow was 

 brighter than the general ground, and also a deeper 

 blue. For a short distance on both sides of the 

 shadow the phosphorescence was a little feebler 

 than at a greater distance. 



"This shows that, though the direct rays of the 

 sun by themselves alone would have strongly 

 excited the phosphorescence, yet acting along with 

 the diffused light from all parts of the sky they 

 did more harm than good. 



"The slightly inferior luminosity of the parts, to 

 some little distance on both sides of that on which 



