124 EMISSION OF LIGHT 



Medusae upon a tin plate, he observed that when- 

 ever he struck this plate with another metal, the 

 slightest vibrations of the tin were sufficient to 

 render the animal completely luminous many 

 consecutive times. It has been observed also that 

 these Medusae emit a more intense phosphores- 

 cence when they are placed in a galvanic circuit ; 

 but the electric current must not be too powerful. 



The experiments undertaken by M. Suriray a 

 Havre, by Professor Ehrenberg at Heligoland, by 

 M. de Quatrefages at Boulogne, and by Dr. Ver- 

 haghe at Ostend, have added considerably to our 

 knowledge of the emission of light by Noctiluca. 



All mechanical or chemical agents that bring 

 about a contraction of tissue in these animalcules 

 excite their phosphoric quality. A drop of weak 

 acid or alcohol, a shock given to the glass 

 which contains them, immediately renders every 

 individual luminous. If a few teaspoonfuls of 

 Noctiluca be collected upon a filter, the light they 

 emit is powerful enough to enable us to read at a 

 distance of nine inches and a half. When the 

 bulb of a small and very sensitive thermometer is 

 plunged into this little heap of Noctiluca, it is 

 found, that although these small beings are in full 

 life, not the slightest elevation of temperature can 

 be observed during the emission of their light. 



A curious observation has been made by Pro- 

 fessor Ehrenberg. By submitting his "Mam- 



