164 



O. Barus — Measurement of Fog Particles. 



tinctly visible caustic. Similarly on moving the lamp horizon- 

 tally to either side from the position corresponding to vivid 

 pink color, fig. 3, and axial illumination, the globules become 

 opaque and look like round shining steel beads. The diameter 

 of the beads has but little effect on the axial or the other 

 color.* If the lamp is moved until the field is dark, the plate 

 looks like the starry heavens. These " stars " or foci are seen 

 to be above the drops. They should lie (roughly) -OOOS/'SS to 

 •0015/-33 cm above it. 



After remaining in the plate for some minutes the fixed 

 droplets often become rosette-shaped (apparently), at first (fig. 

 4) showing a mere black spot in the center of the color disc 

 which gradually enlarges to a ring-shaped appearance slowly 

 moving radially out/wards. As a rule, the color is eventually 

 the same on the inside and the outside of the enlarged ring, 

 the ring itself appearing red with black demarcations in the 

 surrounding green field as shown in the figure. On influx of 

 air the structure becomes washed. The small globules when 

 at first deposited never show the same color within and with- 

 out, the former being uniformly red and the latter white. As 

 the rings are not easily produced with a very viscous oil, it is 



® © 



o 



probable that the droplet has here penetrated to the glass and 

 that the oil film is drawn over it by the capillary forces at the 

 common edge of the three media. But the cause of the radial 

 expansion is an interesting subject for conjecture. 



4. Preliminary measurements. — Before adopting the eccen- 

 tric focussing device, many experiments were made to ascer- 

 tain the cause of the uncertainty in catching the drops on 

 the plate when kept in place, seeing that sometimes the pre- 

 cipitation was abundant, while at other times, under the same 

 apparent conditions, drops did not fall. Failures occurred both 

 for high and for low nucleation. From the outset it was 

 improbable that radiation from the outside could affect the 

 result. It was eventually ascertained that on tipping the con- 



* The degree of transparency will depend on the amount of contact of the 

 globule and the oil film, and the floating globules are therefore sometimes 

 quite opaque, at other times partially or wholly transparent. 



