J. AITKEN ON THE NUMBER OF DUST PARTICLES IN THE ATMOSPHERE. 3 



showering down like fine rain. Now, it appeared to me that it might be possible to 

 count the number of drops that fell on a given area. If this could be done, then the 

 problem promised to be a simple one. All that would be necessary would be to fix a 

 small stage or platform inside the receiver, at a known distance from the top, to have 

 the surface of the stage ruled into squares of a known size, and then by simply counting 

 the number of drops that fell on one of the little squares, it would be easy to make 

 an estimate of the number of dust particles in the air tested. 



Supposing, for instance, that the platform was ruled with lines at right angles to each 

 other, and at 1 mm. apart, and suppose the stage was fixed at 1 cm. from the top of the 

 receiver. Then, if say one drop fell on each little square, that would give 100 drops per 

 square centimetre, and, as there is only 1 cm. of air above the stage, there will be exactly 

 100 particles per cubic centimetre of the air in the receiver ; and knowing the amount to 

 which the air was diluted with pure air, and expanded by the pump, the calculation for 

 the number of particles in the air tested is easily made. Such was the plan which seemed 

 to give the greatest promise of success, if the details could be successfully carried out. 



The first thing to which attention was directed was to find the most suitable kind of 

 stage or platform on which to receive the drops, and on which they could be most easily 

 seen and counted. This simple problem took long to solve ; and now that the difficulties 

 are overcome, and the apparatus so arranged that the drops are very easily seen, and 

 quickly counted, it is difficult to see how the time has been spent. The first counting 

 stage tried was a small piece of glass mirror, ruled on the back with fine lines, at right 

 angles to each other, and at 1 millimetre apart. The back of the mirror was covered with 

 black varnish to show the lines clearly. The stage was supported inside the receiver at 

 a distance of 1 cm. from the top, and its surface was examined by means of a magnifying 

 glass. The stage was illuminated by a gas flame placed at a short distance and slightly 

 above it. 



This small mirror gave results which at first looked highly satisfactory, the drops being 

 clearly visible on its surface with the aid of the magnifying glass. But when it came to 

 counting the drops, the unfitness of the arrangement was evident; for over the most of its 

 surface the drops appeared double, and the confusion produced by the reflected images 

 caused its use to be abandoned at once. The next arrangement tried was a mirror made 

 of very thick glass, so that the reflected image might be quite out of focus ; but the result 

 was not satisfactory, as the cross lines on this stage had to be etched on the upper surface, 

 and these lines caused a good deal of light to be reflected from the mirror, and prevented 

 the drops being so easily seen as on the first one. The next attempt was made with 

 microscope glass, on which I deposited silver in the usual way; but though the thin glass 

 brought the drop and its reflected image almost together, yet it had to be abandoned, as 

 all the samples of this kind of glass tried were too rough and full of specks, and only 

 the most highly finished glass is suitable for the purpose. 



After this, glass mirrors were abandoned, and another arrangement tried. Instead of 

 using a mirror lighted from above, the stage was now lighted from below, and a small piece 



