Without burdening the reader for the moment with details to be considered presently, the effect in telescopic vision is that a star of real angular diameter quite negligible, perhaps 0.″001 of arc, is represented by an image under perfect conditions like Fig. 154, of quite perceptible diameter, surrounded by a system of rings, faint but clear-cut, diminishing in intensity outwards. When the seeing is bad no rings are visible and the central disc is a mere bright blur several times larger than it ought to be. 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. 8. Disc always sharply defined, (_a_) long arcs. (_b_) Rings complete all in motion. 9. Rings, (_a_) Inner ring stationary, (_b_) Outer rings momentarily stationary. 10. Rings all stationary, (_a_) Detail between the rings sometimes moving. (_b_) No detail between the rings. The first three scale numbers indicate very bad seeing; the next two, poor; the next two, good; and the last three, excellent. One can get some idea of the extreme badness of scale divisions 1, 2, 3, in realizing that the third bright diffraction ring is nearly 4 times the diameter of the proper star-disc. It must be noted that for a given condition of atmosphere the seeing with a large instrument ranks lower on the scale than with a small one, since as already explained the usual air ripples are of dimensions that might affect a 5 inch aperture imperceptibly and a 15 inch aperture very seriously. Douglass (loc. cit.) made a careful comparison of seeing conditions for apertures up to 24 inches and found a systematic difference of 2 or 3 scale numbers between 4 or 6 inches aperture, and 18 or 24 inches. With the smallest aperture the image showed merely bodily motion due to air waves that produced serious injury to the image in the large apertures, as might be expected.