198 
DK. W. J, RUSSELL OX THE FORMATIOX OF 
Tlie lollowiiiLT tio'uies show the effect which other forms of obstruction have on 
these dust tigures. A glass ring 4 millims. thick and 0'75 millim. high was placed 
at the centre of a square plate, and juoduced no alteration of the cross (tig. 35). 
Then a ihig U5 milliin. liigh was used, and it produced but little effect (fig. 3G); 
i)ut when a ring 3 millims. high was used, then the central part within the ring 
Ijecame to a considerable extent thickened, and much deposit was formed (fig. 37), 
and when the ring was 5 millims. high an even deposit was formed inside the ring, 
but the rays of the cross outside were not affected (fig. 38). The effect of offering 
Fig. .35. Fig. 36. 
obstructions of different kinds to the ffow of these dust currents was further tested 
by supporting from above, instead ol from below, a strip ot glass longer than the 
square plate on which the deposit was to be formed. When this is hung against 
the side of the plate, a dense deposit takes place all along this edge, but when the 
screen extends about lU millims. on both sides beyond the })late, the deposit stops at 
