Plate VI 



Fig. II. Aberration in the spider's eye. The figure represents the images formed by an eye from a 

 perfect square which was placed in eight diflFerent places near the periphery of the field of vision. When 

 a square is placed in the eye-axis its image is free from aberration. 



Fig. 12. This figure represents the comparative acuity of vision in man, and in two hunting 

 spiders. The posterior middle eyes possess the greatest acuity of vision in Lycosa nidicola, and the 

 anterior middle eyes possess the greatest acuity of vision in Phidippus tripunctatus. The black discs 

 represent a row of rods in the retina of Phidippus and Lycosa, and of cones in the yellow spot of man, 

 all equally magnified. The heavy black lines represent the length of the images formed by the same 

 object placed at the same distance from each of the three eyes. In the human eye this image will occupy 

 57 cones, in the anterior middle eye of Phidippus nearly seven rods, while in the posterior middle eye 

 of Lycosa the image is only a little longer than the diameter of a rod. 



