The denominator is the longest perpendicular line that can be drawn between the upper lid and lower lid when they are maximally opened. Common values for 47 arboreal species are in the order of 1.5. They jump to around 2.0 for the 7 terrestrial species tested. Both measures are necessary because of the wide spectrum of species involved in such measurements. 7 Human eyes have the largest SSI in 88 primate species and the eye's outline shows extraordinary horizontal elongation.
Humans are also the only species with completely white sclera's.
Conversely, in other species, the outline of the eye is almost entirely filled with the iris, which is also comparatively darker and larger. Moreover, arboreal primates have lower SSI and WHR measurements than terrestrial ones. All such measurements correlate with body size measures, and also with gaze scanning time. Newborns have lower SSI and WHR values than adults. Diurnal primates have no more pigment in their sclera's when compared to nocturnal primates, so the putative function of glare reduction is less likely to give reason to sir Duke Elder, an otherwise unsurpassed ophthalmological authority.