153 
1888 - 89 .] Mr John Aitken on Dust Particles. 
used to shield the observer as much as possible when a strong light 
is employed. For concentrating the light on the stage a globular 
flask full of water may he used, or a short focussed glass lens, such 
as the condenser of a magic lantern, may be employed. 
General Remarks. 
In beginning work with the apparatus, the first thing to be 
attended to is the counting stage. If bright, it is carefully dusted 
only, but it is generally polished every time before use. This is done 
either with a buff wheel with rouge, and finished on a woollen 
wheel, or it is rubbed on a piece of chamois leather stretched 
on a fiat surface, a very little rouge being used, and the polishing 
being finished with a clean piece of leather. In polishing the stage, 
the rubbing should be all done in straight lines along the engraved 
lines on the plate, as previously directed ; it need not, however, 
be always along the same lines. When properly polished, the plate 
should have a brilliant black appearance, if one may use the expres- 
sion ; this enables the counting to be easily done. The rubbings 
must on no account be done circularly, as is usual in polishing silver, 
nor must it be done in straight lines at any considerable angle to the 
engraved lines, or the rubbing marks will be distinctly visible, 
destroying the perfect blackness of the mirror, and making counting 
difficult. To save the frequent polishing required by silver, platinum 
and other metals are now under trial. 
Having polished the mirror, and been careful to keep its surface 
free from dust specks, we put it into its socket in the top of the 
supporting pipe D, and D is put into its place in the receiver. The 
stage is then adjusted so that the corners of the squares point to the 
light. In that position it should look quite black, and the lines be 
distinctly visible on it. It is probable that all the little squares will 
not be perfect. Owing to imperfections in the metal, &c., little specks 
will be visible in different places ; but as there are more than one 
hundred squares, the most perfect are selected for observing and 
counting the drops. The stopcock on the entering pipe O is now 
connected with the measuring apparatus and closed. The stop- 
cock T is opened, and the pump worked till the water entering by 
T rises to the mark in the receiver ; T is then closed. A slight cir- 
cular shaking is given to the receiver to cause the water inside to 
