DK. W. J. KUSSELL ON THE FORMATION OF 
11)4 
it, no figure, only an even deposit is formed. Now, if in place of a screen extending 
the whole length of the plate a small one he set up, a piece of glass 5 millims. wide, 
for instance, the same kind of action occurs, the plate immediately behind the screen 
is ])rotected, and there a deposit of dust fhrms, of a curious rounded shape (fig. 24]. 
Fig. Fig. 24. 
A still nari'ower obstruction may be used. The efiect produced by a pin fixed against 
the plate is shoAvn in fig. 24, and fig. 25 shows tlie effect of a fine human hair. In 
neither of these cases does the deposit commence at the ol)struction, but a little wav 
from it. A piece of thin wire acts exactly in the same way as a hair. Experiments 
were then made to ascertain what effects altering the position of the pin would have 
on the figure ])roduced, and it was found, that as long as the pin is in contact 
witli the plate, its height above it does not affect the deposit formed. In all these 
