368 



Discoveries of Mailer and others in the 



or less hemispherical, whether the eye be large or small. 

 The visual field of the hemispherical eye {Jig, 67 » a p b) is 



bounded by the prolongations of the diameter of the hemi- 

 sphere a h ; for the eye a oh the visual field is limited by 

 the elongated radii c i and i h ; for the eye still less convex 

 [a n b), by the radii d k and kg ; and the eye of least convexity 

 {a m b) has the smallest field of vision {elf)', the axes of the 

 transparent cones seeming in all cases to be, as here supposed, 

 perpendicular to the surface of the cornea. From this it fol- 

 lows that eyes of equal circumference, but of unequal con- 

 vexity, or forming segments of different spheres, but with 

 equal chords, have a field of vision more extensive exactly in 

 proportion as the angle comprised between the two sector 

 radii is greater. If each of the two eyes forms the quarter 

 of a hemisphere, the internal sides of both being in exact 

 parallelism, the common field of vision of the two ought to 

 equal in its circumference the half of a hemisphere. If the 

 form of the eyes be elliptical, as in the grasshoppers and 

 others, the boundaries of the field of vision will be likewise 

 elliptical, and so with eyes of any other form. 



The mode of progression of the animal is undoubtedly in- 

 fluenced by the circumference, the form, and the position of 

 the eyes. Goetze * covered the compound eyes of a Fespa 

 Crabro with a layer of opaque varnish, and the animal then 

 flew only in a perpendicular direction ; the only one, indeed, 

 in which the stemmata, placed on the upper part of the head, 

 still supplied it with a minute field of vision ; the compound 

 eye of one side being covered with the varnish, the animal 



* Belehrung liber gemeinnutzigt Natur und Lebenssachen, 1794, p. 42. 



