The Sense of Sight in Spiders 



301 



with the brush on both sides. They are then cut separately out 

 of the cephalothorax and suspended, each first on a drop of water 

 and then on one of glycerin-albumen and the image which each 

 forms of a 10 cm. square at a distance of 10 cm. is measured 

 with the aid of the ocular micrometer. These measurements 

 may then be directly compared with those obtained from the 

 sections through the retina. In measuring the distance between 

 the rods I use the highest power only and count how many 

 microns are occupied by ten rods. This is essential since 

 the distances between the rods are apt to vary a little. Besides, the 



TABLE X 



An adult Lycosa nidicola, small individual. Average distance between the centers of the two rods in 



micromillitneters 



rods are larger and farther apart at the periphery, gradually becom- 

 ing smaller and lying closer together toward the center. I do not 

 give a drawing of this but Table X affords sufficient demonstration. 



It seems to me, in view of the strange shape of the retina, that 

 we may form an idea of the acuity of vision in the spider's eye 

 only from images that cover the central part of the retina alone. 

 In order to diminish the possible error, I measured the image of 

 10 cm. as mentioned, but divided the result by ten so as to find 

 the size of the image of i cm. from 10 cm. distance. 



In this way we obtain the following table: 



TABLE XI 



Phidippus tripunctatus 



