1. The retinae of the principal (AM) eyes of jumping spiders contain four layers of receptors, one behind the other with respect to the incident light. The distribution of receptors in each layer has been determined.
2. The deepest layers (1 and 2) cover the whole area of the retina, and have the greatest density of receptors. The minimum receptor separation is 1.7 µm., or 11 min. visual angle. The more superficial layers (3 and 4) are confined to the central region of the retina.
3. In layers 1, 2 and 3 the rhabdomere-containing segments are rod-shaped, and are parallel to the incident light. In layer 4 they are ovoid, and are oriented approximately at right angles to the light.
4. At the first optic glomerulus the primary fibres from each receptor layer appear to terminate in separate regions of neuropile.
5. The focal lengths, radii of curvature and refractive indices of the lenses of the principal and side eyes have been measured. For the principal eyes, estimates have also been made of the diffraction limit, the depth of focus, and the magnitudes of chromatic and spherical aberration.
6. The normal position of the image in the eye was found by ophthalmoscopy. For blue-green light, the best image of distant objects is formed on the next-to-deepest layer (2).
7. The deepest layer (1) is conjugate with a plane about 2 cm. in front of the animal for blue-green light, or with infinity for red light, because of the eye's chromatic aberration.
8. Two theories are offered to account for the retinal layering. Either the spider uses different layers to examine maximally sharp images of objects at different dis tances; or each layer exploits the sharpest image of distant objects, but for light of different wavelengths.
9. Optical considerations indicate that either theory is possible, but the seconds (wavelength) theory is the more probable, because jumping spiders are known to possess colour vision. It predicts that layer 1 receptors contain red-sensitive, layer 2 blue-green sensitive and layer 3 violet-ultraviolet Sensitive pigments.
10. The structural peculiarities of the most superficial layer (4), and the fact that it is not conjugate with any plane in front of the animal for any visible wavelength, suggest that it is not resolving an image, but performing some other function. Reasons are given for thinking that this might be the analysis of the pattern of polarization of skylight.