Abstract
Abstract: :
Purpose: All simple lenses have longitudinal chromatic aberration (LCA). Light of different wavelengths is focussed at different distances from the lens. This, if not corrected, will cause blurring of the retinal image. Because of small depth of field, eyes with short focal lengths and large pupils, such as present in fishes and nocturnal terrestrial animals, suffer the most from LCA. Some vertebrates capable of colour vision have evolved lenses with multiple focal lengths, each of which creates a well-focussed image at the peak of absorbance of each spectral type of cone. Watasenia scintillans is the only cephalopod known to have several, spectrally different visual pigments. We measured longitudinal spherical aberration (LSA) in lenses of W. scintillans to determine whether this invertebrate species also has multifocal lenses. Methods: Squids were captured in Toyama Bay, Japan, and their lenses excised and placed in sea water, to which they are also exposed in the living animal. A thin laser beam was scanned through a meridional plane of each lens from 0 to 0.95 R (R = equatorial lens diameter). 125 beam paths were analysed to determine back vertex distances (BVDs) as a measure of LSA. Using measured LSAs, ray-tracing model calculations were performed to determine the focusing properties of the lenses. From each animal, only one lens was used in the analysis. Results: Eleven lenses of W. scintillans were investigated. The lenses were almost spherical and the mean focal length was 2.35 R. This is in the same range as found in fish lenses. The lenses had some positive LSA, indicated by smoothly decreasing BVD as a function of eccentricity from the optical axis. Discrete multiple focal lengths could not be detected. Conclusions: The lenses of W. scintillans are slightly undercorrected for LSA. Plateaus in the LSA and thus multiple focal lengths, as are present in fish lenses, are absent. The eyes of cephalopods and fishes show many features of convergent evolution. The optical problems associated with colour vision, however, appear to be solved in different ways.
Keywords: 362 color vision • 500 optical properties • 519 physiological optics