It has long been known that the corneas and lenses of some vertebrates are yellow in colour, and therefore affect the spectral quality of the light that reaches the retina. Various functions have been attributed to such intraocular yellow filters, such as the reduction of scattered light, chromatic aberration, and ‘glare’; and also possibly an increase in the contrast of some objects viewed against certain backgrounds (see Walls & Judd, 1933; Muntz, 1972, for reviews). It is of course likely that the yellow filters of different animals serve different functions, but it is also clear that in all cases, by absorbing a proportion of the incident illumination, they must reduce the animal's absolute sensitivity. In agreement with this, there is a general tendency for animals with yellow filters to be diurnal, and occupy brightly lit environments. Among the fishes in particular, there is a tendency for yellow lenses and corneas to be especially common among diurnal groups, which avoid the consequences of the inevitable loss of sensitivity by remaining inactive at night (Muntz, 1973).