62 Trans. Acad. Sci. of St. Louis. 
In obtaining such ionization of the air around the 
grounded anode terminal, the plates of the machine 
should be turned with extreme slowness. There should 
be a gap of a couple of mm. at the machine terminal, and 
the rotation should cease for a moment, if it is thought 
that a spark may pass across the plate. After half a 
minute the velocity of rotation may be increased and the 
spark allowed to pass over the film. This method of pro- 
cedure will give time for the ionization to be brought 
about, and will occasionally give desired results. 
In Fig. C of Plate XXII the pin soldered to the end of 
the terminal wire was so bent, that the rounded head 
made contact at the central point of its rounded head with 
the photographic film. The form of the shadow shows 
that the cause for the fogging effect around this pin-head 
lies in a very thin layer of air at the film. 
Fig. C of Plate XXII also shows that the origin of the 
agency which produces the fogging effect is at the muzzle 
of the discharge channel about midway between the ter- 
minals. Fig. D is a print from Fig. C. Hundreds of 
plates have been used in securing a comparatively few 
specimens which show the dispersion of the disruptive 
discharge at some point between the metallic terminals. 
In no ease has such a dispersion area of ionized air ex- 
tended to the negative pin-head. These ionization ef- 
fects always have their origin at the grounded pin-head, 
although the presence of the negative pin-head is of 
course necessary in order to make the grounded pin-head 
effective. 
Fig. A of Plate XXIII shows one of two exposures 
which have been obtained, in which the disruptive part of 
the discharge is a very small part of the distance be- 
tween the pin-heads. 
In both cases there is evidence that Kiaks was a dis- 
charge towards the negative terminal such as might be 
produced by an issuance of positively (or, perhaps, nega- 
tively) charged particles from the negative end of the 
spark channel. There is slight evidence of such dis- 
charge in other plates, which show the negative discharge 
