258 TEXT-BOOK OF ENTOMOLOGY 



was shown by Grenadier to be untenable, after repeating Gottsche's experiments 

 with the eyes of moths, in which the crystalline cones are firm and attached to 

 the cornea. He was thus able to remove the soft parts, and to look through the 

 cones and the cornea. When the microscope was focussed at the inner end of 

 the cone, a spot of light was visible, but no image. As the object-glass was 

 moved forward, the image gradually came into view, and then disappeared 

 again. Here, then, the image is formed in the interior of the cone itself. 



Exner attempted to make this experiment with the eye of Hydrophilus, but 

 in that insect the crystalline cones always came away from the cornea. " He, 

 however, calculated the focal length, refraction, etc., of the cornea, and con- 

 cluded that, even if, in spite of the crystalline cone, an image could be formed, 

 it would fall much behind the retinula." 



"In these cases, then," adds Lubbock, "an image is out of the question. 

 Moreover, as the cone tapers to a point, there would, in fact, be no room for an 

 image, which must be received on an appropriate surface. In many insect eyes, 

 indeed, as in those of the cockchafer, the crystalline cone is drawn out into a 

 thread, which expands again before reaching the retinula. Such an arrange- 

 ment seems fatal to any idea of an image." 



Lubbock thus sums up the reasons which seem to favor Miiller's theory of 

 mosaic vision, and to oppose Gottsche's view : " (1) In certain cases, as in 

 Hyperia, there are no lenses, and consequently there can be no image ; (2) the 

 image would generally be destroyed by the crystalline cone ; (3) in some cases 

 it would seem that the image would be formed completely behind the eye, while 

 in others, again, it would be too near the cornea ; (4) a pointed retina seems 

 incompatible with a clear image ; (5) any true projection of an image would in 

 certain species be precluded by the presence of impenetrable pigment, which 

 only leaves a minute central passage for the light-rays ; (6) even the clearest 

 image would be useless, from the absence of a suitable receptive surface, since 

 both the small number and mode of combination of the elements composing that 

 surface seem to preclude it from receiving more than a single impression ; (7) no 

 system of accommodation has yet been discovered; finally (8), a combination 

 of many thousand relatively complete eyes seems quite useless and incompre- 

 hensible. " 



In his most recent work (1890) on the eyes of Crustacea and insects, Exner 

 states that the numerous simple eyes which make up the compound eye have 

 each a cornea, but it is more or less flat, and the crystalline part of the eye has 

 not the shape of a lens, but of a " lens cylinder," that is, of a cylinder which is 

 composed of sheets of transparent tissue, the refracting powers of which decrease 

 toward the periphery of the cylinder. If an eye of this kind is removed and 

 freed of the pigment which surrounds it, objects may be looked at through it 

 from behind ; but its field of vision is very small, and the direct images received 

 from each separate eye are either produced close to one another on the retina 

 (or rather the retinulse of all the eyes) or superposed. In this last case no less 

 than thirty separate images may be superposed, which is supposed to be of great 

 use to night-flying insects. Exner claims that many other advantages result 

 from the compound nature of an insect's eye. Thus the mobile pigment, which 

 corresponds to our iris, can take different positions, either between the separate 

 eyes or behind the lens cylinders, in which case it acts as so many screens to 

 intercept the over-abundance of light. Exner finds that with its compound eyes 

 the common glow-worm (Lampyris) is capable of distinguishing large signboard 

 letters at a distance of ten or more feet, as well as extremely fine lines engraved 

 one-hundredth of an inch apart, if they are at a distance of less than half an 

 inch from the eye. Exner substantiates the truth of the results of Plateau's 



