COLD LIGHT—-HARVEY 211 
itself with a black mass of ink; imagine one’s surprise at the dis- 
charge of a cloud of “ fire ” that glows in the sea water for some time. 
What is the use of this remarkable power? Perhaps to frighten or 
blind predacious animals while the squid makes good its escape. 
It is not known with certainty. 
Many fish produce a light of their own, apart from the light of 
luminous bacteria growing on the dead fish. The living fish contain 
organs which in themselves are light-producing, especially forms liv- 
ing in the deep sea. These organs are arranged in rows on the sides 
or bottom of the fish, giving it the appearance of a ship with all its 
port holes illuminated. Sometimes the organ is dangled on the end 
of a long stalk projecting from the head of the fish, a Diogenes of 
the deep in search of an honest meal. 
Some of these luminous organs are exceedingly interesting from a 
structural standpoint because they are veritable lanterns. ‘They have 
been carefully studied by Prof. Ulric Dahlgren °94, who has con- 
tributed much to our knowledge of the histology of luminous animals. 
In many ways they resemble the eye because they have a lens, except 
that the lens in the case of the luminous organ is used for directing 
the light, whereas in the eye it is used for receiving the light and 
converging it on to the retina. The more complicated of these lumi- 
nous organs have not only a lens, they have also a layer of cells 
which contain a shiny material, and this shiny material makes the 
layer act as a reflector, so that when the light is produced in the 
middle of the organ, that which comes back against the reflector is 
shot forward and out through the lens, and all the light is directed 
and concentrated ina beam. Not only does the organ have reflectors, 
it has also opaque screens, in order to protect the tissues of the ani- 
mals from any light which may pass out the side and possibly injure 
cells around the luminous organ. Light—strong light, at least—is 
destructive to living tissue, and where we have an organ in the 
animal producing a light of its own, we have, practically, a very 
strong light, and we find in most cases the organs or tissues pro- 
tected by some kind of a screen. 
There may also be present color screens, which allow only certain 
wave lengths to pass, and so give the light a definite color. These 
have been described in luminous cuttlefish from the depths of the 
ocean. One species has at least three colored luminous organs—a 
blue, a violet, and a reddish organ. An insect from South America 
has not only white luminous organs, but also red ones, and these 
red lights, so it is said, are very conveniently situated at the tail 
of the insect, and the white lights at the head. It is know locally 
as the “ automobile bug.” 
Two luminous fishes found in the Dutch East Indies, in the Banda 
Sea, are of great interest, because they have developed a luminous 
