The Sense of Sight lu Spiders 291 



eyes. Care must be taken, however, not to remove the black 

 pigment ring surrounding each lens on the inside surface of the 

 cephalothorax and forming a sort of iris, as this would make a 

 difference in the measurements. Even in spiders that have 

 been kept for a long time in alcohol the lenses are often still so 

 transparent that one may see the images formed by them, but such 

 lenses are yellow and the images rather poor. If on the contrary 

 the eyes are prepared in the manner just described, it is scarcely 

 possible to give an idea of the beauty of the little images. The 

 lenses are then entirely colorless and transparent and the images 

 render correctly color and line. 



For the study of the fields of vision, each eye together with a 

 little of the surrounding chitin must now be cut out of the cephalo- 

 thorax. The lens is then placed with its inner surface on a small 

 drop of liquid on a slide and in a hanging position examined under 

 the microscope through the slide. By this means three advan- 

 tages are gained. First, the object examined sends its rays 

 through the lens in the normal direction so that the eye of the 

 examiner is substituted for the retina of the spider; second, the 

 observer looks in the direction of the eye-axis or at least very 

 nearly in that direction; and third, the outside surface of the lens 

 remains dry, limited by the air alone, as is the case with the living 

 spider. More difficulty is presented by the fact that the refraction- 

 coefficient of the vitreous body is not known. However it is 

 sometimes possible to prepare an eye fresh with the vitreous body 

 in its natural position and the retina cut off with the aid of a 

 razor. We are then able to measure the image of a scale at a 

 given distance. But since the vitreous body coagulates too 

 rapidly to be used in the study of the maximal and minimal 

 angles of vision, we have to use in its stead a drop of water and 

 also of some liquid possessing a higher refraction-coefficient 

 than the vitreous body. With the latter we obtain a somewhat 

 larger image than in reality and may oi'^restimate the acuity of 

 vision. Such a liquid I found in a mixture of equal parts of pure 

 glycerine and egg albumen. The results obtained by the use of 

 water we may then employ for control, to guard us against the 

 opposite extreme of an i/«^^restimation of the acuity of vision. 



