2q8 Alexander Petrunkevitch 



the Peckhams that spiders are able to discriminate between 

 colors. Neither can we know whether gradations of light and 

 shade are the same for the spider as for us nor how great the 

 amplitude of the light wave, which would be required to produce 

 the same effects as in us. 



Nevertheless we do know that an image is formed in each spider 

 eye; we do know that the four pairs of images differ from each 

 other in size; we do know that the more rods covered by the image 

 the more detail can be perceived by the eye. We may thus work 

 on a fairly safe basis. 



Let us first examine the images as they appear under the 

 microscope. When a black square is placed under the microscope 

 so that the axis of the eye is perpendicular to the center of the 

 square, it is not possible to detect any spherical aberration by 

 common means. But if we place the eye so that the black square 

 lies considerably to one side of the eye-axis, the aberration at 

 once becomes appreciable. For the accompanying drawing (Fig. 

 1 1) an eye of Lycosa was put so that it was a little outside the center 

 of the microscopic field and the axis of the eye formed a more or less 

 sharp angle with the slide, while the black square, each side of 

 which was 5 cm. long but having one side prolonged into a straight 

 black line, the whole made with chinese ink on a plate of milk- 

 white glass, w^as absolutely parallel to the slide. The Zeiss draw- 

 ing table was carefully arranged beforehand so that a small square 

 placed under the microscope and drawn with the aid of the Abbe 

 apparatus, gave on paper a perfect square. Then drawings of 

 the image in the spider's eye of the black square were made from 

 different positions of the eye, obtained by revolving the table of 

 the microscope on its axis. It is clear from the drawing that in this 

 case the base and one side of the square are especially distorted. 

 Is the spider's vision then distorted of all things that he out of the 

 axis of the eye ^ It is impossible to know but I believe that the 

 spider forms a true idea of objects, first because the distortion in 

 each eye of a pair is in the opposite sense to that in the other eye of 

 the same pair, thus offsetting it, and second, because the retina is 

 not a plane but is of very complicated form differing in different 

 eyes and for different species. Generally speaking we may compare 



