ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 497 



compound eyes), and contains the key to its principle. Only those 

 rays of light can affect the eye which fall on it in the radial direc- 

 tion, i. e. in the direction of the long axes of the crystalline cones. 

 Each retinula (retina of a single unit) receives a cylindrical bundle 

 of light-rays from every visible object ; exactly the same amount of 

 an object is taken in, at whatever distance it is viewed, so the effect of 

 motion is not produced by an increase in or reduction of the amount 

 which is seen of a moving body. 



The action of direct sunlight on insects is evidently, from their 

 sensitiveness to it, of great importance to them. Seeing that the 

 angle which the rays proceeding from the orb of the sun make on 

 reaching the earth is on the average 32', the smallest angle of 

 vision for a unit of any insect's eye being probably more than 10' 

 (39' is the lowest known to the author, viz. in an JEschna), the single 

 image of the sun would be spread, at the most, over three unit-eyes, 

 and, at the least, over one ; while the minute unit of the human eye, 

 having an angular distance of 10" only, can take in T ^ 7 of the sun's 

 disk, and thus the disk occupies in the retina a surface 192 units in 

 diameter, and covering about 27,000 rods. The amount of light 

 which can be received directly by the facetted eye from the sun is far 

 less than that received by the human eye, in fact only from -^Vu" *° 

 STirw °f * no amoun * ; received in the latter case. The bearing of this 

 striking fact on the habits of the insect is difficult to see, but it may 

 be asserted that the insect's eye is thus well provided against the 

 effects of a too intense light, while its sensibility to minute grades of 

 illumination from terrestrial objects remains incontestably one of its 

 most important properties. For by far the greater amount of the 

 impinging light is absorbed by the epidermic structures, and owing 

 to the spherical curvature of the eye, the rays which reach it coincide 

 in direction with the optical axes of but a few of the units, and so 

 but a small portion of the receptive nervous region is affected by 

 them ; thus only the x^-tfrjWo P art 0I> * ne sun's disk is perceived by a 

 single eye. This view is supported by Grenadier's opinion that it is 

 the median (i. e. direct and unrefracted) rays of the pencil which 

 strike a facet, which are the most important. The perception of an 

 object in all its dimensions and of its relation to surrounding bodies 

 cannot be learned, as it is to some extent in our own case, during the 

 short life of the insect. Some idea of the character of the insect's 

 vision may be gained from the observed fact that the natural impulse 

 of the insect is to court the darkness (e. g. the lower sides of leaves, 

 the shade of grass, &c), in order to avoid observation ; their well- 

 known delight in brilliant illumination forming merely an episode 

 in their life of caution. Probably they are to some extent subject to 

 optical delusion; thus when the sun suddenly goes behind the 

 clouds, the surrounding objects, before so brilliantly illuminated, 

 would appear to be at a greater distance, owing to their loss of light. 

 To ascertain the relations of objects with regard to the surrounding 

 space is the most important function of this organ in these animals, 

 and especially the relation of distance from the eye itself; these 

 ends are attained by the comparatively large angular distance which 



Ser. 2.— Vol. II. 2 L 



