ON THE STRUCTURE OF NOCTILUCA MILIARIS. 49 



which the rays seem to radiate in all directions ; if we depress 

 still further, this luminous spot also disappears, and we see 

 nothing but the uniformly-illuminated layer of balsam. If 

 the aperture of the object-glass is small, the luminous rings 

 are either not seen at all, or fainter, in proportion to the 

 extent of the aperture ; and, on depressing the object-jflass, 

 the opaque spots disappear at once, and we cannot trace the 

 rays of the ring down to the lowest point. For the sake of 

 greater accuracy, I made these observations with the same 

 object-glass, the aperture of which gradually diminished by 

 stops. It is also necessary that, during the different trials, we 

 should always have the same focus ; tliis cannot be done by 

 looking at the opaque depressions, but by bringing any other 

 fine mark or scratch on the surface of the layer always first to 

 its exact focus. Candlelight is preferable to dayliglit. I did 

 not use a condensor, but the ordinary plane-reflecting mirror, 

 being anxious to study the phenomena under their most simple 

 conditions. At certain inclinations of the mirror, the dots 

 become much elongated, so that one can easily understand 

 how rows of dots, if close together, produce the aj)pearance of 

 lines. I have thus not only proved that the theory advanced 

 by Dr. Griffith is untenable, and contrary to fact, but also 

 shown, or made it at least probable, how the contrast of light 

 is produced which renders the marking visible. 



On the Structure of Noctili'ca miltaris. By Thomas H. 

 Huxley, F.R.S. 



Among the many striking and beautiful appearances pre- 

 sented by the Ocean, there is none, perhaps, which has 

 more attracted the attention both of the naturalist and of the 

 casual observer, tlian the silvery, sparkling, phosphorescent 

 light, which may often be seen on dark nights, illuminating 

 the track of every boat and defining the contours of the 

 waves as they break upon tlie shore. 



After long serving as a fertile subject of doubt and dis- 

 cussion, it is now well known that this luminosity proceeds 

 from many sources ; in the main, from living invertebrate 

 animals — Protozoa, Polypes, Medusae, Annelids, Crustaceans, 

 &c. Among these again, the chief and most important part 

 is played, as was first shown in the middle of the last century 

 by M. Rigaut, and again in 1810 by M. Suriray,* by a sin- 

 gular and anomalous creature of very simple organization, the 

 Noctiluca miliaris. 



* See Qitat refuges, I. c. I regret that I have not access at this moment 

 to M. Suriray's paper. 



VOL. 111. K 



