304 Alexander Petrunkevitch 



These figures certainlv show the great superiority of the human 

 eve over even the best eve of spiders. This is especially manitest 

 from the fact that even if we use a drop of Canada balsam with 

 its 1.535 refraction-coefficient, for the suspension ot the eye we 

 still find the hmit of vision for the AME of Phidippus to be 6' 

 and for the PME of Lvcosa to be 45'. At the same time we must 

 not forget that the minimal angle of vision in the spiders is in 

 inverse proportion to the maximal angle and that with the use 

 of the Canada balsam the measurements would show a reduction 

 in the fields of vision from 40° in the AME of Phidippus to about 

 onlv 30^. On the other hand the measurements made with the 

 use of glvcerin-albumen show that a female jumping spider 

 placed at a distance of 30 cm. from its mate, would still give a 

 sharp image covering a sufficient number of rods in the eye ot the 

 male to be reco^ized bv him — a result which stands in con- 

 formitv to the experiments of the Peckhams. Perhaps this 

 relation between field of vision and acuity of vision was responsible 

 for the fact that the eves of spiders did not attain the acuity of 

 vision of the human eve. 



We have alreadv seen that the smaller the diameter of a spiders 

 eve, the smaller the image it forms. We have also seen that this 

 does not applv with exactness to different eves of the same eve- 

 group but measurements show that for the same eyes in spiders 

 at different ages the ratio holds good as far as it is possible to 

 ascertain with the methods used. The younger the spider the 

 smaller the images in its eves and the question arises: are the 

 eves of spiderlings as sensitive as those of adults or does the power 

 of distinction grow with increasing age r Spiderlings that are 

 readv to leave the mother can be prepared as I prepared the 

 adult spiders and image and retina mav be studied in the veri' 

 same eve. But this cannot be done with spiderhngs taken out 

 of the cocoon so we mav base our conclusions on the admission 

 onlv, that the ratio between diameter of eve and image holds 

 good for these spiderlings also. This is indeed probable since 

 I have proved that such a ratio exists in all ages beginning \N-ith 

 the oldest spiderlings. In ever%' case I examined for comparison 

 purposelv the mother spider with her voung ones, to avoid errors 



