The Light of Fire-Flies. 
[October, 
50 
The only case at all similar is that of strontium sulphide, 
rendered phosphorescent by light and by an increasing tem- 
perature. As the temperature rises the less refrangible 
rays appear in the spectrum, but, at the same time, accord- 
ing to E. Becquerel, the more refrangible rays disappear. 
On examining the luminous organs with a little attention, 
it is found that when the light begins to appear the central 
and interior portion alone is luminous. It is only when 
the light becomes very brilliant that it reaches the peri- 
pheric stratum in which Robin and Laboulbene have proved 
the presence of a number of fine oily drops. These savants 
think that the peripheric stratum does not produce the 
light, and merely reflects that produced by the central 
portion of the organ. However this may he, it is curious 
to remark that the red rays do not appear until this 
peripheric layer becomes luminous. 
The authors have next examined the photo-chemical, or, 
as the common expression is, the photographic power of the 
light. Although the spectrum extends but little towards 
the violet they tried its action upon plates rendered sensi- 
tive with the gelatino-bromide of silver. After some 
fruitless attempts they arrived at satisfactory results. A 
bit of lace-work of blackened paper was placed before the 
sensitive plate, which was then exposed to the light of 
one of the luminous organs placed distinctly above the 
middle of the design. The other organ sent its rays almost 
parallel to the plate, which it illuminated a little on one 
side. The insect was placed at the distance of o’02 metre 
from the plate. 
By reason of this proximity the illuminated field was ol 
small extent, and scarcely went beyond the borders of the 
design, save on the side illuminated by the second organ. 
In order to obtain a decisive result the plate was exposed 
for an hour, but the action was so intense that a much 
shorter exposure would evidently have been sufficient. 
Five minutes were afterwards found sufficient, and the 
authors think that they could have obtained results in a 
still shorter time if the death of the specimen had not put 
an end to the experiments. 
The photographs show that the light of the Pyrophorus 
has a very intense chemical action, especially if we con- 
sider that these organs, though brilliant, emit but a very 
small quantity of light, as was proved by photometric 
experiments. 
The light of the Pyrophorus renders calcium sulphide 
phosphorescent after five minutes’ exposure. The phos- 
