1890 - 91 .] Mr J. Aitken on a Simple Pocket Dust-Counter. 47 
to and spread themselves over it, more or less, but there is no 
internal reflection, and only a slight external one on the convex 
side next the light. 
The problem then came to be — Could it not be possible to prevent 
the drops adhering to and spreading themselves on the glass ? In 
some trials I got encouragement to suppose this might be possible 
by coating the surface of the glass with some substance that would 
repel the water. The manner of testing this was to coat a clean plate 
of glass with the substance under trial, allow a shower of spray to fall 
on it, and examine the drops with a lens. In this manner many 
substances were tried, but the best results were got with paraffin-wax 
and refined beeswax. These substances were put on the glass, 
and then rubbed off till their presence could scarcely be detected. 
Glass so treated was found to act exactly like silver ; the spray 
rested on the surface in little round balls, and showed the internal 
reflection well. 
These substances were then tried in the dust-counter, on small 
silvered glass counting stages, and it was found that they did 
perfectly under certain conditions, but it was difficult always to 
secure these conditions with the very small pieces of glass. The 
treatment was tried in practice for a< time, but it was found to be 
troublesome, as it did not always produce the desired result. The 
plan was therefore abandoned as it was not thought good enough, 
nor sufficiently simple, and certain in its action, to be put into the 
hands of most observers. 
Experiments were therefore begun in another direction, and trials 
made of the effect of illuminating the stage from beneath. If we 
place a mirror underneath the glass stage so as to reflect the light 
of the sky through the stage, no satisfactory result is obtained 
owing to the general glare of light. However, I have fortunately 
succeeded in lighting the stage from beneath in such a way 
that not only are the drops visible, but they are seen with a 
distinctness far superior to anything yet obtained, even with silver 
in its best condition and best lighting. Not only so, but a very 
low degree of illumination is sufficient to show the drops clearly. 
One great advantage of this is, that observations can be made in 
early morning and late evening, when the light is far too feeble for 
working with silver. 
