48 Proceedings of Royal Society of Edinburgh . [sess. 
This method of illumination is shown in the drawing of the 
instrument (see figs. 1 and 3). The counting stage is made of 
glass, and is illuminated from beneath, the light being reflected 
upwards by what we might call a spot-mirror, which is simply an 
ordinary mirror with a black circular space in the centre. This 
enables the drops to be illuminated by means of a slightly oblique 
light, while an image of the black spot covers the field of the lens. 
The result is the drops are seen shining brilliantly on a nearly black 
field, and are counted with great ease. 
After satisfying myself of the value of this method of working, a 
difficulty presented itself. I had an ordinary micrometer made of a 
size suitable for the counting stage. This micrometer was made by 
a professional maker of these instruments ; but on fitting it into the 
dust- counter, the method of illumination was found to be so powerful 
and trying, that it brought out all manner of imperfections and 
blemishes on the micrometer which were not seen with a magnify- 
ing glass and ordinary illumination. The cross lines on the 
micrometer looked rough, with a crystalline glistening appearance, 
and there were so many specks on its surface that working with it 
was very difficult, as few squares were free from spots, which were 
apt to be counted as drops. The makers of the micrometer were 
therefore written to about these imperfections ; their reply was that 
“ they had done their best, carefully selecting the glass, &c., and that 
they thought it would be difficult to get a better instrument.” If 
better could not be got, I felt that the value of the new arrangement 
would be greatly decreased. I therefore determined to attempt the 
manufacture of micrometers myself, to see what could be done. A 
piece of patent plate-glass was procured, this was cut into suitable 
sizes and very carefully examined with a strong lens, while it was 
illuminated by means of a spot-mirror. After finding a fairly good 
piece in the glass, any specks which were on its surface were tested 
with a pointed piece of soft wood, and if they were not found to be 
removable, the part was rejected and the search continued. In this 
way a few pieces were obtained large enough for the purpose, and 
perfectly free from specks. These perfect pieces were marked off 
on the glass, cut out, and fine cross lines at one millimetre apart 
were engraved on their surfaces ; after which they were turned 
into little circular discs of the required diameter. 
