1890 - 91 .] Mr J. Aitken on a Simple Pocket Dust-Counter. 43 
filtering apparatus attached to the pocket instrument. With the 
removal of the air-pump valves its use would he inconvenient, and 
it is not a necessary part of the apparatus. For viewing the 
counting stage the magnifying lens M is used. A common single 
lens of about two-centimetre focus does very well for the purpose. 
It is lighter and less expensive than a compound one. The 
lens is mounted in a tube which slides into another tube, this 
larger tube is attached to a disc of brass of the same diameter as 
the top of the receiver. This disc has a flange all round it of such 
a size that, when cut so as to give it a spring, it grasps the top of 
the receiver firmly, but in such a way that it can be easily lifted 
off. This is necessary, as the inside of the glass cover of the 
receiver often gets dewed, and the easiest way of removing the 
condensed moisture is to lift off the cover carrying the lens, and 
hold the finger on the glass to heat and evaporate the moisture 
from the inside surface. 
Before proceeding to describe the improvements in the counting 
stage, it will he as well to describe the manner of using the new 
instrument. The first thing to he done is to see that the inside of 
the receiver is wet. If it is, then examine the inside surface of the 
glass cover of the receiver, and see" if it is free from condensed 
moisture, which would interfere with a clear view of the stage. If 
it is not clear, take off the metal cover and hold a finger on the 
centre of the glass plate till it begins to clear, and then replace the 
cover. Too much heating should be avoided, as it gives rise to 
trouble with the counting stage. Now examine the surface of the 
counting stage, and see if it is free from specks. If it is not 
satisfactory, take it out and clean it with a piece of clean cloth. 
Care is advisable in doing this to see that the cloth is perfectly 
clean, as otherwise the stage will look dirty and streaky in the 
humid atmosphere in the receiver. If the stage is simply dewed, 
then touch the underside of it with a finger to heat it slightly. 
If the finger is not quite clean, put a thickness of cloth or other 
protection over it. If these two glass surfaces are in good order, 
the instrument is ready for making a test. 
If it has been necessary to take the counting stage out of the 
receiver to clean it, then this will have admitted much impure air, 
aud as there is no filter to enable us to fill the receiver with pure 
