168 POSITION OF THE IMAGE. 



cone. This does not, indeed, always occur ; but even in 

 such cases the image does not coincide with the posterior 

 end of the cone. Grenacher repeated the experiment 

 of Grottsche with moths. Here the crystalline cones 

 are firm, and are attached to the cornea. Thus he was 

 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 had 

 endeavoured to make this experiment with the eye of 

 Hydrophilus (the great black water-beetle), but the 

 crystalline cones always came away from the cornea. 

 He, how^ever, calculated the focal length, refraction, 

 etc., of the cornea, and concluded 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, 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 (Fig. 100), the crystalline cone 

 is drawn out into a thread, which expands again before 

 reaching the retinula. Such an arrangement seems 

 fatal to any idea of an image. 



Moreover, for definite vision by the formation of an 

 image, it is necessary that the eye should possess some 

 powder of accommodation for different distances. It is 

 obvious, from Fig. 76, that no distinct vision would 

 be given unless the receptive surface follows the 

 line a' V c\ But the position of this surface will 



