REPORT ON THE DIATOMACEiE. 13 



The sea on dark nights is lighted up under the strokes of oars, in the track of boats and 

 ships, along the shores, and among the breakers, and it is known that this attractive 

 phenomenon is chiefly due to myriads of Noctiluca?, Peridinia, and other small pelagic 

 forms ; yet these organisms emit a momentary splendour only when they are disturbed by 

 an extraneous body or by the mechanical action of the waves, although the highly 

 specialised apparatus in Nyctvphanes norvegica 1 is apparently an evidence that in some 

 animals this power is under the influence of the will. Mr. Murray also informs me that 

 he has observed very many animals emit phosphorescent light in the stillest waters without 

 any apparent external stimulant. 



Granting that the phosphorescence of abyssal animals is a normal characteristic, it 

 must still be proved that it is not only sufficient to dispel the darkness so effectively 

 as to explain the presence and development of visual organs, but that it is capable of 

 so great actinic action as to determine the vivid colouration above referred to. 



Just as the bathymetrical limit of marine life, which was laid down at 300 fathoms 

 by Professor Edward Forbes, is now no longer accepted, so the distance to which light can 

 penetrate may prove to have been misunderstated hitherto, and should, in the interest of 

 truth, be re-examined by physicists. 



It is stated that in the passage of light through sea water the first rays to be absorbed 

 are the calorific, then the luminous, and lastly the actinic or chemical, which are precisely 

 those that have most influence on colours and on chlorophyll and consequently on 

 vegetation in general, and attempts have been made to fix the limit of the penetration of 

 light by observing the gradual descent of a white object, and by noting the moment when 

 that object could no longer be discerned. But even although the distance thus obtained be 

 more than doubled, and the observer placed in a condition to be in no way influenced by the 

 external light, such observations are not very reliable. 



Since several indications of the influence of light in marine abysses now exist, an 

 explanation of the manner in which it can penetrate to even very great depths should be 

 sought after. 



An interesting observation may here be noticed. When two French aeronauts were 

 recently crossing the English Channel, and when at a great height above its surface, they 

 were struck by the circumstance that its bed could be distinctly seen, and that all the details 

 of the irregularities of its bottom could be traced. It may be said that the depth of this 

 narrow belt of water is not to be compared to that of the great oceans, but assuredly even 

 its depth could not be seen by an observer near the surface, and perhaps it is not less than 

 the limit hitherto placed on the distance to which light can penetrate. 



1 Narr. Chall. Exp., vol. i. p. 743. 



