Silk is often recommended, but some silk is exceedingly objectionable in texture,--old silk, perfectly soft to the touch, is perhaps as good as anything.

If the dust which has fallen on the glass is at all gritty, the glass will suffer by the method of cleaning commonly adopted, in which the dust is _gathered up_ by pressure.

The proper method is to clean a small space near the edge of the glass, and to _sweep_ from that space as centre.

In this way the dust is _pushed before_ the silk or wash-leather, and does not cut the glass.

It is well always to suspect the presence of gritty dust, and adopt this cautious method of cleaning.
The two glasses should on no account be separated.
In examining an eye-piece, the quality of the glass should be noted, and care taken that both glasses (but especially the field-glass) are free from the least speck, scratch, or blemish of any kind, for these defects will be exhibited in a magnified state in the field of view.