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  Pyramidal Crystal Face Numbers

Pyramidal crystal face numbers are an extension of those for plates and columns.

Top and bottom hexagonal basal faces, if they exist, are numbered 1 and 2.

The middle prism faces are 3 to 8.

The top pyramidal faces are 13 to 18 and the lower pyramid faces are 23 to 28

A 9° odd radius halo is made by 3-26 rays i.e. rays entering the middle prism face 3 and leaving through lower pyramid face 26.

The wedge angle is 28° compared with the 60°
paths 3-5 giving the more common 22° halo.

4-27 or 5-28 or 7-24 rays are equivalent to 3-26 but the path description is always simplified to the lowest face number where possible.