gn 
| Accompanying | Place of | 
Date | Colours | halos “observation | Reference 
| Be i | 
21 | 1906, Oct. 8 | conspicuously li- | C 22° ‚ Zutfen Onw. 
vely colours | 
28 | 1911, March 8, faint red ee Renesse x 
29 Oct. 1 | red | de Bilt 5 
30 | 1914, Dec. 29 | red | CZA red, blue; de Bilt N 
. | P; pillar 
31 | 1890, March 3 all the colours of | C 22°; P; CZA Parc Saint La Nature, 
the rainbow rainbow colours Maur | 18, Sp. 238; 
| 1890 
Compare also: Nrs. 5, 6a, 7, 9b, 12, 20, 22. 
f. Circumzenithic arc. 
82. AOM, Od. ds valk colours to’ UTA Zutfen ‚ Onw. 
‚ violet | | | 
33 Nov. 15 | red, yellow, green | C 22° Munnike- 2. 
| | buren 
Compare also: 1, 15, 30, 31. 7 
IW. The refraction of hght in ice-crystals of a refracting angle 
of 60°. | 
We may confine ourselves to the phenomenon as it presents 
itself in one plane brought through the eye and the sun. By a 
rotation of this plane about the line eye-sun the circular phenomenon 
will be generated. It will also be allowable to consider exclusively 
those crystals whose refracting edge is at right angles to the plane 
chosen, seeing that the colours can only take rise on the inner edge 
of the halo, where the light which moves perpendicularly to the 
axis of the crystals chiefly contributes to the formation of the circle. 
In order to deduce the colour which will be seen in a given 
direction it is necessary to determine the intensities of the various 
spectral colours in that direction: from these the resulting colour 
can be calculated. 
For an angle of incidence 2, 
in the hexagonal prism PCQH 
(fig. 1; only the faces required 
for the construction are shown) 
the passing beam is confined 
within the rays I and II. In 
the direction of a given angle 
of deviation D there are two 
different beams of widths 45 
