The varying appearance of the star image is a very good index of the quality of the seeing, so that, having a clear indication of this appearance, two astronomers in different parts of the world can gain a definite idea of each other’s relative seeing conditions. To this end a standard scale of seeing, due largely to the efforts of Prof. W. H. Pickering, has come into rather common use. (H. A. 61 29). It is as follows, based on observations with a 5 inch telescope.
STANDARD SCALE OF SEEING
1. Image usually about twice the diameter of the third ring.
2. Image occasionally twice the diameter of the third ring.
3. Image of about the same diameter as the third ring, and brighter at the centre.
4. Disc often visible, arcs (of rings) sometimes seen on brighter stars.
5. Disc always visible, arcs frequently seen on brighter stars.
6. Disc always visible, short arcs constantly seen.
7. Disc sometimes sharply defined, (a) long arcs. (b) Rings complete.