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   Crystal face Numbers


We can describe halo ray paths easily and precisely if crystal faces have a standard numbering scheme.

Basal (end) faces are always numbered 1 and 2.

Prism side faces are 3 to 8.
Plate crystals have face 1 uppermost.

A circumzenithal arc has ray paths of 13. This is sequence of faces the rays pass through. Paths 14,15, 16, 17 and 18 are the same, we quote the path with the lowest number equivalent face.
A left hand sundog has a 35 path, the right hand is 37.


Column oriented crystals
A tangent arc is 35.
A supralateral arc is 31.

Parry crystals have face 3 uppermost.

Lowitz oriented crystals have the Lowitz axis passing through the edges between faces 4 and 5.