Page:Scott's Last Expedition, Volume 1.djvu/403

1911] experiments to illustrate; consideration of these facts and refraction of light striking crystals clearly leads to explanation of various complicated halo phenomena such as recorded and such as seen by us on the Great Barrier, and draws attention to the critical refraction angles of 32° and 46°, the radius of inner and outer rings, the position of mock suns, contra suns, zenith circles, &c.

Further measurements are needed; for instance, of streamers from mock suns and examination of ice crystals. (Record of ice crystals seen on Barrier Surface.)

Rainbows.—Caused by reflection and refraction from and through drops of water—colours vary with size of drops, the smaller the drop the lighter the colours and nearer to the violet end of the spectrum—hence white rainbow as seen on the Barrier, very small drops.

Double Bows—diameters must be 84° and 100°—again from laws of refraction—colours: inner, red outside; outer, red inside—i.e. reds come together.

Wanted to see more rainbows on Barrier. In this connection a good rainbow was seen to N.W. in February from winter quarters. Reports should note colours and relative width of bands of colour.

Iridescent Clouds.—Not yet understood; observations required, especially angular distance from the sun.

Auroras.—Clearly most frequent and intense in years of maximum sun spots; this argues connection with the sun.

Points noticed requiring confirmation:

Arch: centre of arch in magnetic meridian.

Shafts: take direction of dipping needle.

Bands and Curtains with convolutions—not understood.