1. Pollak L. W. &Murphy T.,Sampling of Condensation Nuclei by means of a mobile photo-electric counter. Archiv für Meteorologie, Geophysik und Bioklimatologie, Wien (in the Press).
2. —A Portable Aitken Nucleus Counter, Meteorol. Office, Air Ministry, London (Instruction F. 826766/28), 1928.
3. The use of conducting glass has been suggested to us by Assistant ProfessorC. B. A. Mc Cusker of the School of Cosmic Physics.
4. Scholz J.,Ein neuer Apparat zur Bestimmung der Zahl der geladenen und ungeladenen Kerne. Zeitschr. für Instrumentenkunde, Berlin, 51, 505–522 (1931).
5. The instruction quoted in Reference (2) and alsoJ. Aitken (On Improvements in the Apparatus for Counting the Dust Particles in the Atmosphere, Proc. R. S. E., Vol. XVI, 1888–89 andOn a Simple Pocket Dust-Counter, Proc. R.S.E., Vol. XVIII, 1899–91) restrict this limit to five droplets, on the average, to each square. The reason given in the first ofAitken's papers mentioned above is “that if more than 5 drops fall per square mm there is too much dust, not only because when the number is much above 5, there is a difficulty in counting them before they evaporate, if the stage be slightly hot, but also because, with so large a number, we cannot be quite sure that all the particles have been thrown down. Suppose, for instance, that 10 drops fall per square mm; if we now admit only filtered air, and again make an expansion, we shall find that some drops will make their appearance, showing that some particles have escaped the first condensation. It has, therefore, been the practice to limit the maximum number of drops to 5 per square mm. With that number no drops appear on a second expansion being made”. It appears that this statement ofAitken is considered incorrect nowadays since it is the generally accepted practice “to add up all the counts on subsequent expansions until no more droplets appear” (H. Landsberg,Atmospheric Condensation Nuclei. Ergebnisse der Kosm. Physik, 3, 183, 1938).