262 Proceedings of Royal Society of Edinburgh. [sess. 
In order to find the number of drops in this height of air the 
following plan has been adopted. Underneath and concentric with 
the microscope is mounted a tube 5 cm. long and 4 cm. diameter. 
This tube is provided with a bottom and a cover ; these are both 
fixed to an axis parallel with the axis of the tube, so that by 
turning a handle both top and bottom can be slid sideways, and 
the tube closed or opened at top and bottom simultaneously, when 
desired. In the cover is a small opening corresponding to the lens 
of the microscope, and in the centre of the bottom is fixed a 
micrometer illuminated by a spot-mirror. When the top and bottom 
are turned aside the tube is open at both ends, and the air can 
circulate freely through it. On quickly turning the handle both 
top and bottom are closed, and the micrometer by the same move- 
ment is brought under the microscope, and all the drops that fall 
out of the 5 cm. of air over it are counted on a known area. 
This ' instrument was only completed just when the fogs were 
about over for a season, and no satisfactory readings have been as 
yet obtained. It was, however, thought advisable to give this pre- 
liminary note at present, as the season for fogs has gone for a time, 
and it will give an opportunity for any one wishing to make obser- 
vations in this w T ay being prepared for the coming season. 
It may be mentioned that the instrument first described, may be 
found useful for observing the larger particles of dust in the atmo- 
sphere. If it is exposed anywhere with the stage horizontal, the dust 
that settles on the glass can be distinctly seen with the lens. If the 
spot-mirror is applied to a microscope it gives an illumination very 
suitable for examining the smaller particles of dust. The very 
small particles, which are quite invisible under the ordinary form of 
illumination, shine out brilliantly when illuminated by the spot- 
mirror. The spot-mirror has also been found useful in the micro- 
scopic examination of delicate objects other than dust particles. 
