TOM AND THE MAN-OF-WAR. 181 



light up the sea all around." Taking a tall 

 specimen-glass, the doctor filled it from the sea 

 of fire and placed it on a thwart where all could 

 see it. 



" Now you can see what makes the light," 

 he said, pointing out numbers of round animal- 

 culse. " They are Noctilucce the same that we 

 saw at Marquesas. See how the light changes. 

 Sometimes we catch a blue or yellow gleam, and 

 then it deepens to a rich green." 



The boat slowly drifted to shoal water again, 

 and now the scene below them was still more 

 animated. Here a large jelly-fish was moving 

 about in a basin formed of leaf and branch 

 corals, throwing a beautiful light among the 

 branches, lighting up the homes of the zoophytes 

 and making the fishes cast dark shadows. Scores 

 of delicate Medusae moved up and down, or in 

 and out, with as many different motions, each 

 gleaming with a subdued, steady light. 



" They are like satellites revolving around a 

 larger planet, are they not ? " said the doctor. 

 " They may well be called the light-houses of 

 the sea, as one of you suggested." 



