1890-91.] Mr J. Aitken on a Simple Pocket Dust-Counter. 49 
Two methods of engraving these lines have been tried, and both 
of them give much better lines for the purpose than is obtained by 
the usual method of engraving micrometers. One method is to 
cover the glass with beeswax, and draw the lines with a fine 
needle point, and then etch with hydrofluoric acid. The lines 
obtained by the use of ordinary hydrofluoric acid are not very 
suitable, as they require to be of some breadth before they are 
visible with the spot-mirror illumination, and they then show as 
bright glistening lines. The mixture known as “white acid,” how- 
ever, gives a fine line with just that degree of white visibility which 
makes them appear clear without glancing and distracting the 
attention. In this manner the micrometer which is at present in 
use was prepared, and it has been found in every way satisfactory. 
The vapour of hydrofluoric acid also gives good results. In etching 
these lines, trial must be made with the acid and a piece of the 
same glass to find the correct time the micrometer requires to be kept 
in the acid to etch to the required depth, the trial pieces being 
tested under the spot-mirror illumination. The difficulty of draw- 
ing these lines with a diamond is, that when they are made strong 
enough to be easily seen, they have always bright spots on them. 
It will be observed from the drawing that these micrometers or 
counting stages are made of thick glass. The object of this is to 
prevent any speck, or anything adhering to the under side of the 
glass, interfering with the clearness of the field. The thickness of 
the glass puts them so much out of focus that they do little harm. 
There is then, therefore, no real barrier to the use of these micro- 
meters, only the glass must be selected when under the illumination 
of a spot-mirror. It may be remarked here that the spot-mirror may 
be found useful for other purposes. It gives us a powerful means of 
detecting flaws in lenses, &c. The surface of a new lens when 
examined by means of it looks so full of imperfections that it seems 
scarcely possible it can give a perfect image, while the imperfec- 
tions must give rise to the dispersion of a good deal of light. 
The other method of engraving the lines on the glass, which has 
been tried and found to give good results, is to cover the glass with 
very fine emery powder, wetted with turpentine, and scratch the 
lines with a needle point ; or better, to tip the needle with a 
little diamond bort. The fineness of these lines can be made 
VOL. XVII. 21/2/91 
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