The Sense of Sight ni Spiders 293 



in the eye, how many centimeters are visible. Since the dis- 

 tance between the spider's eye and the scale equals 10 cm. a 

 simple calculation will give the value of the angle in question, or 

 it is still simpler and entirely sufficient for our purpose, to draw on 

 paper an isosceles triangle, the base of which must be as many 

 centimeters long as are visible on the image of the scale in the 

 eye and its height 10 cm. The angle can now be directly meas- 

 ured. When the angles have been measured for all the 

 eyes, they are represented on the drawings in each projection, 

 showing the field covered by each eye. Optically the angle 

 depends upon the curvature of the lens and the refractive coef- 

 ficient of the substance of which it consists. But what is of 

 interest for us here is the general fact that the larger the spider's 

 eye, the smaller, as a rule, is its field of vision. 



If we compare the drawings of corresponding projections in 

 different spiders after the maximum angles of vision have been 

 introduced, we cannot fail to recognize the remarkable relation 

 between the particular life of the spider and the position of its 

 eyes. In order to make this clear I must state here that which I 

 shall prove farther on, that the larger the spider's eye, the sharper 

 its vision or power of distinction. Let us begin with an examina- 

 tion of the projection on the horizontal or foundation plane (Fig. 

 i). We see that the largest eyes in Phidippus are the anterior 

 middle ones covering a field of 40° each or both together about 

 55°, owing to the fact that their axes are a little divergent. Each 

 of the anterior side eyes also covers a field of 40° or both together 

 83°, /'. e., more than the entire field covered by the AME.^ But 

 the ASE are considerably smaller than the AME. The minute 

 eyes of the second row, the posterior middle eyes, cover a field 

 of 62° and the PSE one of 48° or the whole eye-group covers 

 about 240° of the horizon. The projections on the other two 

 planes (Figs. 4 and 6) show in addition that the PME and the 

 PSE guard chiefly the sides of the spiders, leaving about 52° in 

 the vertical plane and more than 80° at the back on the dorsum, 

 entirely unguarded. This is the only direction from which the 



' These are abbreviations commonly used by arachnologists. AME stands for anterior middle eyes; 

 ASE for anterior side eyes; PME for posterior middle eyes, and PSE for posterior side eyes. 



