A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. I15 



the evaporation should be no less than during the anterior 15 or 30 seconds of 

 subsidence during which the fog particles were caught. This would seem to be 

 especially true where the particles persist for several minutes after photography, 

 as is usually the case. Nevertheless, it is not impossible that the first precipitate 

 prepares the plate (by evaporating partially) for the subsequent precipitation. 

 The subject will be resumed in connection with definite restalts below. 



Results. 



15. Photographic plates. — Unfortunately, it is impossible to reproduce the 

 photographic negatives obtained, except in certain cases; for not only are the 

 fog particles frequently too small, but slides suitable for the measurement of 

 number are indistinct in relation to diameter. To obtain a plate in which the 

 particles actually stand out is naturally a matter of chance, as this will depend 

 on many conditions (light, evaporation, focus, etc.) beyond the observer's con- 

 trol in expeditious work. Curiously enough, the plates obtained for fog par- 

 ticles condensed on the persistent nuclei produced by the X-rays in dust -free 

 air are the best of my series, and are therefore reproduced. 



To measure the sizes of the particles on the plates a filar micrometer of 

 low power was used. To count them, the slide was divided off into fields of 

 convenient size, and the number then enumerated under a lens, taking all the 

 fields in clear focus in succession. 



16. Tabulated results. — The following tables show the general results for 

 about 50 photo- microgra]:)hs. The kind of nuclei used are given in the first 

 column, the coronas and their angular diameters (s/t,o, nearly) in the second 

 and third. The mean diameter, d, of fog particle measured from the photo- 

 graph and the limits of d observed, follow. The next two columns show the 

 number of nuclei, n, ol^tained from the photograph by the equation of this 

 chapter, § 5, and the observed limits of n for the different fields counted. The 

 last columns are explanatory and usually show the film of oil or varnish used to 

 catch the precipitate and the character of the plate. 



17. Remarks on the tables. — The use of rough oil films (tallow, paraffine, 

 wax, etc.) is naturally tmsatisfactory, but the particles are easily recognized 

 (No. 3). Plates Nos. 4, 6, 22, though very perfect as photographs (large camera 

 magnification) showed a permanent impression which lasted 6-12 hours, as 

 already stated. In No. 8 only a few particles were caiight. Nos. 9 and 10 were 

 taken with a ^-incli objective, and thotigh good in themselves were hard to 

 obtain, and showed the advisability of the weaker objective (^-inch), favorable 

 to shorter times of exposure. No. 11, and particularly No. 22, gave evidence of 

 the occurrence of "craters" (Fig. 8) in the varnish film left after the evapora- 

 tion of the fog globule. As a whole, the photographs on the X-ray nuclei 

 are the most successful, and in No. 23 in particular the particles stand out on 

 the photograph. In No. 31 particles were deposited on the plate during the 

 vortical motion occtu-ring on influx of air while in No. 32 the motion of the 

 particles (film too liquid) shows streaks on the plate. 



