74 On the Phosphorescence of Marine Invertebrata. 
to the external atmosphere was essential to the result. But on 
the contrary, it is found by observation that in a vase of seawater 
containing the Noctilues, the bed of these animals that collects 
at the top of the vase is equally luminous in every part. 
3. Color of the light.—When the Noctiluce are in full vigor of 
life in quiet water, the color is a clear blue. ut on agitating the 
water, or in the waves of the sea, the light becomes nearly or quite 
white or like silver sprinkled with some greenish or bluish spangles. 
4, Intensity of the light.—M. de Tessan states that in some 
tropical seas, the phosphorescence is so bright from the breaking 
waves, that he could read ordinary type at a distance of fifteen 
paces. The light from the Noctilucee cannot compare with this. 
At the head of the cove of the “ Pare aux Huitres” at Boulogne, 
it was not possible to tell the hour with a watch, when the waves 
were breaking at the observer’s feet. With a tube 15 millimeters in 
diameter, in which the Noctilnce formed a bed at the surface nearly 
20 mm. thick, the figures of a watch face could be read; but strong 
agitation of the tube was necessary, and it was requisite to hold it 
close to the glass of the watch. Four to five teaspoonsfull of Nocti 
luce were collected in a filter, and on producing the phosphores- 
cence by this means, the hour could be told at the distance of a foot. 
5. No disengagement of heat sensible to a thermometer accom 
panies the phosphorescence.—'T his fact was established by placing 
the bulb of a thermometer in the Noctiluca water while it was 
quiet, and then giving it a shake to produce the phosphorescence. 
he experiment was varied in different ways, 
6. The hight of the Noctiluce may be produced over the whole 
surface of its body or only a part of it.—After a violent agitation, 
the Noctiluce retain the phosphorescence for some time, so that it 
may be studied at leisnre. With a lens magnifying 6 to 8 diame 
ters, it is easy to see that while some of the Noctiluce are phos- 
phorescent throughout, others are but partially so. in the figure 
given above, one of the animals is light over its whole surface; 
