C. Barus — Measurement of Fog Particles. 165 



densation chamber, after the fog had formed so that the subsi- 

 dence would reach the plate obliquely, a precipitate would 

 usually appear. Again, an oblique current within the chamber 

 and passing across the plate usually produced a deposit. Hence, 

 as the drops actually exist within the fog, success in bringing 

 them down upon the plate is conditioned by very close iso- 

 thermal adjustment of the plate to its surroundings, added to the 

 advantages gained from incidental and favorable air currents. 

 Thus a little time must always elapse before the drops persist 

 at the plate, and therefore the droplets from a shallow capsule 

 do not appear. Using a film of mica as a plate the result was 

 the same, and it is useless to attempt to enumerate the drops 

 by this method. Those which fall are carried in by grazing 

 air currents, while no drops are obtained from the fiducial 

 space under the objective. Cf . § 5. 



Nevertheless, the measurement of the diameters, d, of the 

 drops obtained by the given method without modification is 

 an excellent test of the results obtained elsewhere by computa- 

 tion. The factor of the ocular micrometer described above 

 was *002 cm per turn of the screw or -00004: cm per scale part of 

 the drum divided into 50. The extent of plate covered by the 

 breadth of the spider lines was about 'OOOS 0111 . The finest par- 

 ticles are of about this diameter, so that such measurements 

 must at best be much inferior to photography with a scale 

 attachment. I will pass over the tabulated results here, merely 

 stating that the coronal color with its diameter, s (chord of 

 a radius of 30 cm ), was observed when the eye and the source of 

 light (Welsbach mantel seen through a small circular hole) 

 were at distances 85 cm and 250 cm , respectively, from the center 

 of the condensation chamber. The exhaustion was usually to 

 a pressure difference of I7 cm , but this is of no significance when 

 diameters are alone to be measured. The particles were col- 

 lected by tipping the chamber, sometimes in large numbers, 

 but at other times sparsely distributed without apparent cause. 

 Nuclei were conveniently obtained from burning charcoal. 

 Both floating and fixed globules were examined with strong 

 microscopic illumination. It was difficult to retain a clear image 

 without frequently removing the plate as the adjustment for 

 focussing the plate within the chamber had not yet been adopted. 



Floating globules were sometimes observed in the act of 

 coalescing ; but this is much rarer than the passage of a float- 

 ing droplet* over a fixed one without interference. A distinct 

 central red area, shading off into darkness, was seen even in the 

 floating globules when axially illuminated by intense light. 

 The larger drops were often metallically green. The colors 



* Multitudes of fog particles are often seen moving in opposite directions 

 across the field and turning about normally at the edges of a liquid film. 



