VOYAGE TO ALGERIA. 173 



My assistant, Thorogood, occupied a boat, fastened as 

 usual to the davits of the ' Urgent,' while I occupied a 

 second boat nearer the stern of the ship. He cast the 

 plate as a mariner heaves the lead, and by the time it 

 reached me it had sunk a considerable depth in the 

 water. In all cases the hue of this plate was green. 

 Even when the sea was of the darkest indigo, the green 

 was vivid and pronounced. I could notice the gradual 

 deepening of the colour as the plate sank, but at its 

 greatest depth, even in indigo water, the colour was still 

 a blue-green. 1 



Other observations confirmed this one. The 

 ' Urgent ' is a screw steamer, and right over the blades 

 of the screw was an orifice called the screw-well, through 

 which one could look from the poop down upon the 

 screw. The surface-glimmer, which so pesters the eye, 

 was here in a great measure removed. Midway down, 

 a plank crossed the screw-well from side to side ; on 

 this I placed myself and observed the action of the 

 screw underneath. The eye was rendered sensitive by 

 the moderation of the light ; and, to remove still further 

 all disturbing causes, Lieutenant Walton had a sail and 

 tarpaulin thrown over the mouth of the well. Under- 

 neath this I perched myself on the plank and watched 

 the screw. In an indigo sea the play of colour was 

 indescribably beautiful, and the contrast between the 

 water, which had the screw-blades, and that which had 

 the bottom of the ocean, as a background, was extra- 

 ordinary. The one was of the most brilliant green, the 

 other of the deepest ultramarine. The surface of the 

 water above the screw-blade was always ruffled. Liquid 

 lenses were thus formed, by which the coloured light 

 was withdrawn from some places and concentrated upon 



1 In no case, of course, is the green pure, but a mixture of green 

 and blue. 



