902 Alexander Petrunkevitch 



On dividing the size of the image by the distance between the 

 rods, we find that the image of a hne i cm. long (measured in an 

 eye suspended on a glycerin-albumen drop) occupies respectively 

 38 rods in AME, 8 rods in ASE, 2 rods in the PME and 7 rods 

 in the PSE. One square centimeter at a distance of 10 cm. gives 

 an image that occupies 38- = 1444 rods in the AME, 8- = 64 

 rods in the ASE, 2' = 4 rods in the PME and f = 49 rods in 

 PSE. With other words the front middle eyes in Phidippus are 

 by far the most sensitive, then come the front side eyes, then the 

 eyes of the third row and last, the posterior middle eyes. 



In Lycosa nidicola the difference in the sizes of the eyes is less 

 than in Phidippus and consequently the difference in sensitive- 

 ness or power of distinction is less. I examined several specimens. 

 Since in the one used for the measurements which I give here the 

 image of 10 cm. at 10 cm. distance did not coincide with the 

 divisions on the scale on my ocular micrometer, it was necessary, 

 in order to avoid error, to arrange the experiment somewhat 

 differently. The bristol board containing the centimeter scale 

 was brought nearer to or farther from the eye until a number of 

 centimeters not fewer than five, came to occupy a certain number 

 of divisions on the scale. Thus I found that at a distance of 37 

 cm. the image in the PME of 6 cm. occupies exactly 5 divisions 

 of the micrometer scale. Each division being equal to 0.0164 

 mm., it follows that the image of i cm. w^ould occupy 0.0164 X | 

 = 0.0136 mm. Since the distance between the centers of the 

 rods in the center of the retina in this specimen equals i3/<, it 

 follows that I sq. cm. placed at a distance of 37 cm. from the PME 

 will form an image occupying 4 rods (2 X 2). The image of 7 

 cm. placed at a distance of 36 cm. from the posterior side eye, 

 occupies 5 divisions of the scale or the image of i sq. cm. equals 

 0.0117- mm. and occupies 4 rods, the distance between the centers 

 of these being 12^; but this image does not occupy the rods com- 

 pletely. The image of 9 cm. placed at a distance of 30 cm. from 

 the AME, occupies five divisions of the scale or the image of i 

 sq. cm. at 30 cm. distance equals 0.009^ mm. The distance 

 between the centers of the rods in this eye is 9//, /. e., the image 

 occupies 4 rods (2 X 2). 



