The effects of stray light upon retinal pigment densitometric measurements were evaluated by adding specified amounts of stray light to fundus reflections. To allow for interpolations and extrapolations, a computer simulation was devised and validated against the empirical data. The results demonstrate that measured density is greatly decreased by increases in stray light but the time constant of pigment regeneration is little affected.

The retinas of placental mammals appear to lack the large number and morphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the cone sheath and by labeling of cone outer segments with monoclonal anti-visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone subpopulations in placental mammals. Besides a dominant rod population (max. = 400,000/mm2) four subtypes of cones (max. = 3000/mm2) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A second group of single cones lacking oil droplets was labeled by OS-2 antibody. The topography of these cone subtypes showed striking anisotropies. The COS-1 labeled single cones without oil droplets were found all over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemisphere. The population of OS-2 labeled cones was also ubiquitous although slightly higher in the upper hemisphere (200/mm2). The COS-1 labeled cones bearing an oil droplet, including the principal member of double cones, were concentrated (800/mm2) in the inferior, non-tapetal half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The additional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence in the area centralis, and the correlation with the non-tapetal inferior hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.

This paper responds to Philipona and O'Regan (2006), which attempts to account for certain color phenomena by appeal to singularities in the space of “accessible information” in the light striking the retina. Three points are discussed. First, it is unclear what the empirical significance/import is of the mathematical analysis of the data regarding the accessible information in the light. Second, the singularity index employed in the study is both mathematically and empirically faulty. Third, the connection drawn between their findings and some data from the World Color Survey is lacking in quantitative analysis in places where it is needed. The difficulties raised prevent Philipona & O'Regan's conclusions from being accepted.