502 On the Luminosity of the Sea. [Oct., 



On the 24th of May, lying in Simon's Bay, Cape of Good 

 Hope, the water was similarly luminous. The weather was fine, 

 wind W.N.W. light, bar. 30*04, thermom. 60°. On examining 

 the water closely I found that, as before, the luminous effect, though 

 soft, subdued, and apparently uniform, was really due to innumer- 

 able small sparks, and on bringing the water to the light, I found 

 numerous Noctilucae in it, precisely similar to those observed at 

 Singapore. They were not, however, in sufficient numbers to have 

 produced all the light, for in a wine-glassful of water there were on 

 an average not more than a dozen Noctilucas. But besides these 

 bodies, there were a great number of motes in the water, many of 

 which on closer examination appeared, by their rapid jerking loco- 

 motion, to be minute Entomostracous Crustacea. They were so 

 minute, that by the imperfect light on board ship I long tried, 

 in vain, to secure one to place under the microscope. Besides 

 these were some larger species of Entomostraca. 



Nocru.uc.ffi. 



The Noctilucae measured from sh to sis of an inch in diameter ; 

 they were of a pale greenish colour when seen with the naked eye, 

 closely resembling Yolvox in appearance, but with a much less active 

 movement. They had, however, powers of locomotion, though the 

 means were not apparent under the microscope. They had a dark 

 nucleus, usually irregular, but in some cases spherical and well 

 defined. Their circumferential outline was very faint, and their 

 general aspect very variable. A kind of slit appeared to extend 

 through two-thirds of the body, from which faint lines radiated, 

 usually having a double outline, and not reaching the circumference 

 of the sphere, but often terminating in large round granular bodies 

 of various sizes. The whole body was studded with oil globules of 

 various sizes, which strongly refracted the light ; but slight move- 

 ments, which appeared to be taking place in an almost imperceptible 

 manner, soon changed the whole aspect of any individual Noctiluca 

 while under observation, so that the description or drawing of one 

 minute did not answer for the next. Each Noctiluca had a large 

 curved ciliuni projecting beyond the body, and apparently taking 

 its rise from the nucleus. This form of luminosity, although very 



