The Probable Reason
Garry Holloway1 was the first to suggest a meaningful explanation for this: he suggested that, because there is a small difference between pavilion main & half facet slopes (less than 2º in a typical round brilliant), this may cause one to be dark when the other is bright – producing contrast in the gem’s image. Studies of human optical response2 say that this is attractive to viewers; it may be why the people Tolkowsky polled chose the proportions they did.
Fig.1 Simulation of Tolkowsky proportions with 80%-deep halves, using Vasiliev/s software3 at 15º angle of light obstruction by viewer’s head
Obstruction of Reflection by Halves
Fig.2 shows the diamond chart of ‘Faceting Limits’ reversed to match MSU and GIA plots (pavilion vertical) and limited to show only the region of primary interest.
Fig.2: ‘Faceting Limits’ chart reoriented and half limits added
Fig.3: color lines = ‘brightest’ per GIA & MSU
by Bruce L Harding, circa 2004, 2008 Aug 11
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