558 ON THE FORMATION OF DEFINITE FIGURES BY THE DEPOSITION OF DUST. 
agency was the cuiTents of air creeping over the edge of the plate, the dust being 
deposited where the drift was slowest; hut the sharpness of the patterns and the 
cleanness of the lest of the plate are made more mtelhg’ible by the existence of 
Aitken s deal layer ol drifting air through which the dust from above has to 
fall hefoie leaching the jilate. It will be observed that the explanation recj^uu'es that 
the dust has had time to fall completely out of the layer when it was travelling’ 
iindeineath the plate, but has not had time to fall through it to any extent from 
above wlien the layer was above the plate. The thinning out of the clear layer in 
tlie wake of a pin or hair, owing to eddies or broken motion, as described by 
Mr. Aitken, throws light on the features of the deposit thus produced by revealing 
that it IS denser near the centre of the plate, because the dust has had more time to 
fall through this thinned-out layer of clean drifting air ; yet the persistence of the 
effect when the obstacle is far removed from the edge of the plate remains verv 
remarkable.] 
P9AUG. 1903 
