Letter from William Crookes, FRS, to James Mackenzie Davidson Having looked into the matter of the Lenard tube, informs Mackenzie Davidson that the aluminium window was about 1 or 2 millimeters in diameter and fixed in place by an organic cement. Notes that the tube is likely to be useless to Mackenzie Davidson in these conditions, as the presence of organic matter makes it almost impossible to get a high vacuum. 3 November 1913