DEFINITE FIGITEES BY THE DEPOSITION OF DUST. 
197 
at the centre is formed, and when the pin was only 2 millims. above the plate and 
still 3 millims. from it, increase in the deposit occurred. In all these cases with the 
pin supported from above much less deposit was formed than when the pin was 
pointing upwards. 
If the pill he bent at a right angle, it produces on the plate a deposit similar in 
form and amount to tliat produced by a vertical pin at the same distance from the 
plate. 
This action of any neighbouring body on the dust deposit is shown by any rough 
edge which the plate itself may have. If, for instance, a glass plate be used and it 
has been cut in the usual way, in addition to the figure which is dependent on the 
shape, there will he certain lines of deposit darting out in different directions ; these 
are produced by small splinters of glass attached to the edge. Fig. 30 shows this on 
an oblong glass and fig. 31 on a circular glass. If the edges of tlie plate he carefully 
ground, then these lines of deposit cease to be formed. Fig. 32 shows a square glass, 
two of whose edges were left rough and the other two were ground. 
There are many curious alterations in the forms of the figures produced by placing 
on the plate obstructions to the flow of these lines of dust. For instance, taking 
again a square plate, if a strip of glass 1 millim. high and 1 millim. wide be placed 
across one corner of the plate and then the cross be developed, it has no effect, the 
cross forms as if no obstruction were there, but if the strip be 7 millims. high, then a 
marked effect is produced. In front of the strip the ray retains its usual form, hut 
on the other side and round the centre there is a great widening-out of tlie ray and a 
slight banking-up of the dust against the sides of the glass strip. This effect ol the 
obstruction strip is shown in fig. 33. If the strip be even 20 millims. high it acts in 
the same kind of way. If a strip 5 millims. high and 30 millims. long be placed 
parallel with the edge of the plate, and nearly at the centre, the cross is altered in a 
remarkable way, shown in fig. 34. 
