Letter from WSC to Admiral of the Fleet Sir Ernle Chatfield, First Sea Lord on anti-submarine methods, particularly Nichol Prisms, advocated by Prof F A Lindemann [later Lord Cherwell]. These prisms would enable air observers to see through water by cutting out glare - stating that they had been abandoned after the Great War as the waters of the North Sea and Atlantic were greenish and opaque. Asking if they might be more useful in the clear waters of the Mediterranean [carbon].