282 Alexander Petrunkevitch 



severed. The third plane which I call the vertical or transverse 

 plane and which intersects the two other planes at right angles, is 

 geometrically determined by these. After the cephalothorax has 

 been prepared in this manner and thoroughly dried in a warm 

 place, the cover-glass upon which it is fixed is laid upon the center 

 of a square glass plate, which is at the same time the center of a 

 circle drawn upon the surface of the plate with radii forming angles 

 each of which measures ten degrees. If we now use an eye-piece 

 having two lines intersecting in the center at 90°, we can readily 

 measure the angles that the eye-axes form with the plane of sym- 

 metry. To know how this is done we must remember that the 

 eyes of spiders have each a lens, the outer surface of which forms 

 part of the surface of a sphere. When we look at a lens in the 

 direction of the eye-axis, it appears to us as a circle while if we 

 look as it under an angle of less than 90°, its outer surface appears 

 to us as two curves intersecting each other at two points. If we 

 place the eye in the center of the microscopic field and move the 

 eye-piece until one of its lines falls upon these two points of inter- 

 section of the curves of the lens and the other passes through a 

 point midway between them, then the latter line represents the 

 projection of the, eye-axis on the horizontal or foundation plane 

 and the angle that it forms with the plane of symmetry can be 

 read directly from the scale. More exact results would of course 

 be obtained by using a goniometer ocular such as is made by 

 Zeiss, but even the arrangement I have described here, the ocular 

 with intersecting lines, gives an error of not more than a few 

 (3 to 4) degrees. When the angles have in this way been measured 

 a drawing is made with the aid of an Abbe drawing apparatus 

 of the entire eye-group and the eye-axes are then drawn in, in 

 accordance w^ith the data given by the measurements. We obtain 

 in this way a correct figure of the projection of the eye-group on 

 the horizontal. or foundation plane. In order to gain a clear pic- 

 ture of the position of the eyes it is necessary to make two other 

 projections, one on the plane of symmetry and the other on the 

 vertical plane. To accomplish this I put a small drop of beeswax 

 in the center of the circle on the glass plate into which the edge 

 of the cover-glass holding the cephalothorax is pressed. This 



