PRODUCTION OF LIGHT 421 



second species is named Photoblepharon palpebratus . The 

 luminous organs in these fishes is a movable disc below the eye 

 which can be withdrawn into the orbit and is then covered by 

 a flap of skin like an eyelid, situated above the organ ; when 

 the organ is exposed the edge of the disc extends to the edge 

 of the pupil so that the vision of the fish is not obscured and 

 the light is prevented from passing through the coats of the eye 

 by the dense pigment with which the inner surface of the organ 

 is covered. The luminous organ is thus like a dark lantern 

 placed immediately beneath the eye and the light can be ex- 

 posed or covered at the will of the fish. When the disc is cut 

 out from the head of the fish it retains its luminous power for 

 several hours, and it is put on a hook and used as a luminous 

 bait by the native fishermen, who are extremely ingenious and 

 energetic. It has been supposed that these fishes were abyssal 

 but this is not the case ; Anomalops, called by the natives ikan 

 leweri laut, swims in shoals at the surface of the sea ; Photoble- 

 pharon, distinguished as ikan leweri batu, lives singly among 

 the rocks. Obviously the luminous organ can only be of ad- 

 vantage as a bait in the dark, and in the living fish likewise the 

 habits must be nocturnal. 



The power of emitting light occurs also in some of the 

 shark order, namely, in several species of the family Spina- 

 cidae to which our common spiny dog-fish Acanthias vulgaris 

 belongs. As long ago as 1 840 Bennett described the phosphor- 

 escence of a specimen of a species of shark or dog-fish called 

 Isistius brasiliensis which he observed alive on a whaling-ship. 

 The emission of light lasted for three hours, when the animal 

 died and the light ceased. Bennett's description is as follows : 

 " The entire inferior surface of the body and head emitted a 

 vivid and greenish phosphorescent gleam, imparting to the 

 creature by its own light a truly ghastly and terrific appear- 

 ance. The luminous effect was constant and not perceptibly 

 increased by agitation or friction. The only part of the under 

 surface of the animal which was free from luminosity was the 

 black collar around the throat ; and while the inferior surface 

 of the pectoral, anal and caudal fins shone with splendour, their 

 superior surface, including the upper lobe of the tail-fin, was in 

 darkness, as also were the dorsal fins, back and summit of the 

 head." Another observer in i860, also called Bennett, states 



