DEFINITE FIGURES BY THE DEPOSITION OF DUST. 
201 
occurred, but it was still limited to the corners (fig. 44). When the distance between 
the plates was 4 millims., a further inroad of dust took place, and when 5 millims., the 
centre is the only part without deposit, but from the entrance of the dust being 
principally at the corners, a rough cross, formed by absence of dust, and pointing to 
the centre, is distinguishable (fig. 44 a), and at 7 millims. there is an even deposit. 
In the next set of experiments the covering glass, in place of being as large as tlie 
lower glass, was only a strip 14 millims. wide and 190 millims. long, and 1^ millim. 
thick. It was supported on vulcanite pillars which did not influence the depositions 
Fig. 45 Fig. 46. 
of the dust. When this strip was 1 millim. above the plate no dep3slt took place ; 
when 2 millims. above the plate a small' amount occurred, and this was at a distance 
of 7 millims. from the edge of the pla'^e, and of a curved form, of course, under the 
strip (fig. 45). The strip was now raised to a height of 3 millims., and the amount of 
deposit not only increased, but receded further from the edge of the plate, and wa-i 
2 D 
VOL. CCI.-A. 
