250 DEPTHS OF THE OCEAN 



metal cube without any base ; it is intended to cover tightly 

 the lower cube to which the photographic plates are fastened. 

 On the left the apparatus is seen closed, with the cubes suspended 

 at the top of the frame, the smaller one inside the larger. In 

 this position the apparatus is lowered into the water. A 

 small messenger is sent down the line and releases the inner 

 cube, which drops to the bottom of the frame (see the middle 

 figure). The plates are thus exposed. After a certain time a 

 larger messenger is sent down, releasing the large cube, which 

 falls like a shutter over the plates, as seen in the figure on the 

 right. The apparatus is then ready for hauling up, and the 

 cubes are taken out of the frame into the dark-room for develop- 

 ment and change of plates. 



In all previous photometric apparatus for use in the sea the 

 plates were hermetically closed behind a strong glass pane, in 

 order to shield them against the great pressure, but in the 

 photometer here described a totally different principle was 

 applied. The gelatine-film was covered with a glass plate and 

 inserted into a small envelope of thin caoutchouc, with a square 

 opening in front through which the light is admitted. The 

 envelope with the plate was then placed on one of the sides of 

 the inner cube, and the corresponding brass frame was screwed 

 on tightly. The water could penetrate both outside and inside 

 the cube, so that there was the same pressure on both sides of 

 the film and the glass cover. The rubber envelope would be 

 pressed tightly on to the glass plate, so that no water could enter 

 and spoil the film. By this arrangement the apparatus might 

 be exposed to any pressure without any special protection, and 

 it was used at various depths down to 1700 metres without a 

 single plate being cracked or spoilt by water. 



Highly sensitive pan-chromatic plates (4x4 cm.) were 

 employed in the experiments — the windows being, as mentioned 

 above, 2x2 cm. In several experiments a gelatine colour 

 filter was inserted between the photographic plate and the glass 

 cover. Wratten and Wainwright's three-colour filters (red, 

 green, and blue) admit respectively only a certain portion of the 

 spectrum. This made it possible to study the rays present 

 within the separate fields of the spectrum, as well as the total 

 intensity of the rays. These investigations were carried out in 

 the southern stretch of the cruise, and though time and weather 

 did not allow of many experiments, those that were made gave 

 interesting results. 



Some of the plates exposed are represented in Fig. 172. In 



