C. Bar us — Measurement of Fog Particles. 163 



results were obtained, in a manner to be further detailed 

 below, with the apparatus free from capricious behavior. It 

 is of particular interest that the particles caught on the oiled 

 surface persist as brilliant round globules for a long time (10 

 min. or more) in a saturated atmosphere. They very gradually 

 vanish as a rule, on the readmission of air into the condensa- 

 tion chamber. 



To remove the globules for the next experiment the influx 

 of air is thus not generally sufficient. It is necessary to with- 

 draw the microscope from the condensation chamber bodily 

 and to wave it about a few times in dry air. On returning it 

 to the chamber the plate is then again clean and white. 



At first the plate was oiled by a small flat piece of blotting 

 paper saturated with oil and held on a stem, care being taken 

 to remove all excess. Clean machine oil or ordinary illumina- 

 ting oil, or a mixture of the two, subserved the purpose about 

 equally well. Probably the best method of oiling consists in 

 dipping the plate rotated outward to s' in very hot melted 

 vaseline (to drive away moisture), removing the excess while 

 hot by filter paper, and when cold submerging the plate in 

 petroleum for transparency. With solution of vaseline in 

 benzine, etc., I have been less successful.* A film of varnish 

 dried and soaked in turpentine was used. When drops are to 

 be counted by the method given below, the oil film must be 

 practically solid ; otherwise the capillary forces produce an 

 immediate and often startling redistribution of the precipitated 

 granules, though they may not coalesce. 



3. Behavior of the precipitated droplets. — In case of a 

 petroleum film on the plate, the water droplets were sometimes 

 seen to fall and float on the film, which is positive evidence 

 against spurious droplets. They are usually black and circular 

 in outline, but when the light is intense and axial, they are 

 often colored. Fixed globules were apt to be larger and more 

 irregular and pink or red in color. The color was eventually 

 traced to the chromatic aberration of the objective used. This 

 defect was rather useful in detecting clear globules, but it 

 would be fatal in photography. 



On tipping the microscope so that the light does not pene- 

 trate the vividly colored drops axially, they seem to cast 

 shadows in opposed directions for symmetrical inclinations on 

 both sides, as in fig. 2 ; but the phenomenon is probably a case 

 of refractionf with the shadow beginning at the edge of a dis- 



* It has thus far been difficult to produce an oily film free from flaw and 

 quite glossy under the microscope, where surfaces are apt to be either retic- 

 ulated or fluid. 



f That the effects described are associated with the aplanatic foci of 

 spheres (as I at first supposed) is improbable. The globules lie on the oil 

 film through which nearly parallel light enters. The caustic is not shown 

 in fig. 2. 



