ANNOTATIONS AND ADDITIONS. 59 



one rubs a board with part of a Medusa hysocella, the part 

 so rubbed regains its luminosity on friction with a dry 

 finger. On my passage to South America I sometimes 

 placed' a Medusa on a tin plate. "When I struck another 

 metallic substance against the plate, the slightest vibrations 

 of the tin were sufficient to cause the light. What is the 

 manner in which in this case the blow and the vibrations 

 act? Is the temperature momentarily augmented ? Are new 

 surfaces exposed ? or does the blow press out a fluid, such 

 as phosphuretted hydrogen, which may burn on coming into 

 contact with the oxygen of the atmosphere or of the air 

 held in solution by the sea-water. This light-exciting 

 influence of a shock or blow is particularly remarkable in a 

 "cross sea/' i. e. when waves coming from opposite directions 

 meet and clash. 



I have seen the sea within the tropics appear luminous 

 in the most different states of weather ; but the light was 

 most brilliant when a storm was near, or with a sultry atmos- 

 phere and a vaporous thickly-clouded sky. Heat and cold 

 appear to have little influence on the phenomenon, for on the 

 Banks of Newfoundland the phosphorescence is often very 

 bright during the coldest winter weather. Sometimes 

 under apparently similar external circumstances the sea will 

 be highly luminous one night and not at all so the following 

 night. Does the atmosphere influence the disengagement 

 of light, or do all these differences depend on the accident 

 of the observer sailing through a part of the sea more 

 or less abundantly impregnated with gelatinous animal 

 substances? Perhaps it is only in certain states of the 



