Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

other terrestrial vertebrate. The spectacle scale, being composed of keratin, has a higher refractive
index (n ≥ 1.5) than that of underlying tissues (n = 1.36-1.375 if similar to the cornea) (Valentin 1879a,
1879b), making it a thin lens. Sivak (1977) and Caprette (2005) calculated the power of the whole
spectacle in several colubrids based on measurements of curvature and average refractive index of the
whole spectacle and found the dioptric power of the spectacle to be relatively similar to the lens, in
some cases slightly favouring the lens, in others the spectacle. A lens with such a high relative power
results in a shorter focal length for the optical system, which in turn results in a lower f-number (i.e.
greater retinal illumination) and lower image magnification, all other parameters being equal. This
optical design is frequently seen in nocturnal (Roth et al. 2009) and aquatic or amphibious vertebrates
(Sivak 1976; Northmore and Granda 1991; Brudenall et al. 2008; Walls 1942; Duke-Elder 1958). In
comparison, the ratio of cornea:lens dioptric power in a diurnal iguana is approximately 3:1 (Sivak
1977, calculated based on data from Citron and Pinto 1973), and that of a (mostly) diurnal primate,
Homo sapiens, is 2:1. It would therefore appear that the relative flatness of the spectacle constrains the
snake eye, even that of diurnal species, to a predominantly nocturnal or amphibious optical design.
Gecko eyes do not share this unusual morphology, nor do those of other spectacled lacertilians with
well developed eyes. Rather they recall the eyes of lidded squamates in possessing highly curved
corneas with consequent longer focal lengths.
The question of what anatomical characteristics allow the spectacle dermis to remain as
transparent as possible with maximum transmittance of the visual wavelengths of light remains
unanswered. It is conceivable (and likely) that the composition of the spectacle dermis is similar to that
of the cornea, in which transparency is achieved by the orthogonal arrangement of collagen lamellae
and by maintaining its hydration state within a narrow range (Maurice 1957; Cox et al. 1970; Freegard
1997) through passive and active means (Candia 2004). And as with retinal blood vessels, the spectacle
blood vessel walls are transparent (Mead 1976) such that when constricted they are nearly invisible
and difficult to discern even with slit lamp microscopy (van Doorn, unpubl. obs.).
The transparency of the spectacle scale on the other hand presents a novel problem, since most
keratinous structures are translucent at best. The transparency of spectacle scales varies little with
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