G. Barus — Measurement of Fog Particles. 



161 



might not be made visible directly. The chances of success 

 seemed small indeed, in particular as Assmann* had failed to 

 see the particles with the magnifications of even 400 diameters. 

 But after long trial, the result was eventually accomplished in 

 a way that now seems surprisingly simple. 



The compound microscope, J/, magnifying about 100 diam- 

 eters, is provided with a filar ocular micrometer, n. The objec- 

 tive and the whole lower part of the microscope is submerged 

 in the condensation chamber, being suspended for this purpose 

 from the wide rubber cork, C. All lenses below C are her- 

 metically sealed with wax. The microscope originally carried 

 a rigid stem, r, to which were then attached the plate, s, to be 

 examined, the mirror m, and the metallic disc or shield, d. 

 Afterwards the more flexi- 

 ble adjustment shown in the * 

 figure and described below 

 was adopted. The lower 

 side of p, which is flush 

 with the objective, and the 

 upper side of s are covered 

 with wet blotting paper, the 

 latter being perforated to 

 admit light into the micro- 

 scope through the thin 

 cover glass placed at s and 

 held sharply in focus by a 

 suitable clip. The field 

 within which drops are to 

 be counted is bounded at 

 pleasure by the wires of the 

 micrometer. 



This apparatus was totally 

 unsuccessful. Drops were 

 but rarely seen to fall on 

 exhaustion, while the dew 

 soon gathered on the plate 

 s, in such a way as to be 



easily mistaken for droplets ; for the dew evaporates like the 

 latter when the microscope is removed, and the regularity of 

 the pattern on the plate is the only distinguishing feature. 



Various modifications of this apparatus were then used, 

 among them capsule forms similar to Aitken's, but containing 

 a very thin plate of glass or mica or celluloid slightly raised 

 above the base on pellets of wax. It was supposed that this 

 would counteract the tendency of the drops to vanish by 

 evaporation from the warmer glass surface. Capillary metallic 



* Cf . Arrhenius, Kosmische Physik, vol. ii, p. 487, 1903. 



Am. Joup. Sci.— Fourth Series, Yol. XVII, No. 98. — February, 1904. 

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