PHYSICAL OCEANOGRAPHY 



249 



metres ; Petersen found that in the neighbourhood of Capri a Petersen. 

 plate was influenced by the rays at a depth of 550 metres, 

 Luksch made some investigations in the eastern part of the Luksch. 

 Mediterranean, exposing his plate for fifteen minutes, and found 

 that the limit of the light-rays must be drawn at 600 metres. 

 In these experiments the influence of the collected rays on an 

 ordinary photographic plate was studied. 



In order to make some investigations on this subject in the 



jjy pii 



Fig. 171.- 



On the left, as it is sent down 



-Helland-Hansen's Photometer. 

 in the middle, open for exposure ; on the right, closed and 

 ready for hauling up. 



'Michael Sars " Atlantic Expedition, the author constructed a Heiiand- 



- - - - . . - . . . „. , Hansen's 



photometer. 



new kind of photometer, which is represented in Fig. 171. In ^^"sens 



the centre figure — at the lower part — is seen a brass cube ; the 

 four sides and the top have square " windows," and on each of 

 them a small square frame with a similar window (2x2 cm.) 

 can be screwed fast ; the screws and openings are seen in the 

 figure. The cube rests on a larger brass plate, or rather on an 

 india-rubber mat covering the brass plate. The plate and cube 

 are fastened inside a frame, along which they can be moved up 

 and down. At the top of the central figure is seen a larger 



