PHYSICAL OCEANOGRAPHY 251 



the upper row are seen some results without a Hght-filter at Results at 

 Station 51. The plate on the left (No. 10), exposed for 40 'j'^p^hTluh 

 minutes at 500 metres, was strongly influenced by the rays, and without 

 The next plate (in the middle of the upper row), exposed foj- '^o °"''-*i f^''^- 

 80 minutes at 1000 metres, was also blackened by the light-rays. 

 The third plate was exposed for 120 minutes at 1700 metres, 

 and showed no effect whatever. These experiments were made 

 at noon on the 6th June with a clear sky, and show that a good 

 deal of light penetrates to a depth of 1000 metres — considerably 

 deeper than was previously supposed. The limit of light 



Fig. 172.— Photographic Plates exposed at different depths. 

 The upper row from Station 51, the lower row from Station 55. 



sufficient to influence the plate in the course of two hours lies 

 at a less depth than 1 700 metres. 



The lower row in Fig. 172 shows some plates from Station 

 55, all exposed for forty minutes at a depth of 500 metres. The 

 plate on the left was used without filter, and shows the same 

 strong effect as the corresponding plate from Station 51, in the 

 upper row. The next plate (in the middle of the lower row) 

 was exposed with the blue filter ; an influence of the blue rays 

 is visible on the original plate (a faint Roman V), but not so 

 clearly in the reproduction given here. The right-hand plate 

 in the figure was exposed with a green filter, and shows no 

 effect. A plate with the blue filter needs an exposure six times, 

 and one with the green filter eighteen times, as long as a plate 



