Search Results

Kevin van Doorn and Jacob G. Sivak

Edited by J. van Rooijen

The spectral transmittance of the optical media of the eye plays a substantial role in tuning the spectrum of light available for capture by the retina. Certain squamate reptiles, including snakes and most geckos, shield their eyes beneath a layer of transparent, cornified skin called the ‘spectacle’. This spectacle offers an added opportunity compared with eyelidded animals for tayloring the spectrum. In particular, the hard scale that covers the surface of the spectacle provides a unique material, keratin, rarely found in vertebrate eyes, a material which may have unique spectral properties. To verify this, shed snake and gecko skins were collected and the spectral transmittance of spectacle scales was spectrophotometrically analyzed. The spectacle scale was found generally to behave as a highpass filter with a cut-off in the ultraviolet spectrum where taxonomic variation is mostly observed. The spectacle scales of colubrid and elapid snakes were found to exhibit higher cut-off wavelengths than those of pythonids, vipers, and most boids. Gecko spectacle scales in turn exhibited exceptional spectral transmittance through the visual spectrum down into the UV-B. It is suggested that this is due to the absence of beta-keratins in their spectacle scale.


As a contribution to the study of ecological types among gekkonoid lizards, eye size, expressed as a percentage of rostrum-anus length (percra), was examined. Preliminary tests showed that preservation effects and sexual dimorphism are negligible, that ontogenetic allometry and geographic variation have to be considered, and that external spectacle diameter is proportional to whole-eye size. Species means of spectacle diameter vary: Diplodactylinae (13 spp.), 5–7 percra; Gekkoninae (37 spp.), 3.7–8.7 percra; Sphaerodactylidae (4 spp.), 4.2–5.5 percra. The eye is larger (in percra) in nocturnal than in diurnal species, and independently, is larger in ground-dwelling than in climbing species. The evolution of the latter condition is attributed mainly to the difficulties of visual hunting on the level ground, as compared to scouting from perches. Other hypotheses are also discussed.

Scott Buchanan, Megan McLean and Todd Tupper

for ∼ 12% (94 / 802 m), ∼ 8% (71 / 880 m) ∼ 10% (13 / 455 m) and ∼ 3% (8 / 304 m) of total shoreline lengths at Duck, Dyer, Kinnacum and Spectacle Ponds (these sites are colloquially termed, ‘ponds’ but are actually lakes; Galvin, 2008), respectively. Water was tannin-lignin poor and mildly acidic

Ilona Kovacs, Arvind Chandna, Philippa M. Pennefather, Anthony M. Norcia and Uri Polat

. The inter-observer difference in the measurement of threshold was assessed in 20 eyes of 10 adults (9 normals and 1 amblyope). All participants wore optimal spectacle correction throughout testing, which included monocular distance logMAR acuity, orthoptic examination, and fundoscopy. Procedure. Cards

C.C. Doncaster

longer ones, e.g. during time-lapse sequences, it was kept in position to limit evaporation. Condensation droplets can be prevented from forming on cover- glasses by coating them with a thin film of agar or by wiping them with spectacle cleaner, such as 'Calotherm' or 'Spray Kleen' (Heunert, 1971). Fig

M.A. Georgeson

, using their spectacle corrections, and always fixated a small dark point at the centre of the screen. Each session began with about 20-s inspection of a uniform display, followed by 3- min exposure to the adapting grating whose contrast and location (left or right) were chosen at the start of the

T.S. Aiba and M.J. Morgan

.64 arcmin. Observers were seated with their chins supported by a rest, and wore a spectacle trial frame containing a 1 mm artifical pupil in front of the 157 preferred eye, and an occluder in front of the other. The small pupil was used in an attempt to reduce longitudinal chromatic abberation (cf

Masami Funakawa

(both ramp types) remained 1.0 at all times. Moreover in the second pair, the duration and SOA of both members were co-varied. Six observers participated in this experiment. They all had normal visual acuity, in some when corrected with spectacle lenses. The subject was instructed to align the vernier

Peter C. Dodwell

mapping from E (the physical environment) to I (its internal representation) and the fact that, as every spectacle wearer knows, the mapping is adaptable, there is a further conundrum. Not only can one adapt in the coarse sense of being able to avoid bumping into furniture, recognizing familiar items in

David Westwood and Melvyn Goodale

. 76 , 1439– 1456. Milgram, P. (1987). A spectacle-mountedliquid-crystaltachistoscope, Behavioral Research Methods, Instruments and Computers 19 , 449– 456. Milner, A. D. and Goodale, M. A. (1995). The Visual Brain in Action. Oxford University Press, New York. Milner, A. D., Dijkerman, H. C., Pisella