290 Animal Life and Intelligence. 



be made to give a separate image of such an object as a 

 candle reflected in the mirror of the microscope. If each 

 lens thus gives an image, is not each the focussing apparatus 



of a single eye? But a 

 somewhat more difficult 

 experiment points in 

 another direction. If the 

 facetted cornea be re- 

 moved with the crystalline 

 cones still attached (Gren- 

 acher was able to do it 

 with a moth's eye), and 

 placed under the micro- 

 scope, when the instru- 

 ment is focussed at the 

 point of the cone (where 

 Fig. 37. Eye of fly. the nerve-rod comes), a 



Transverse section through head. (After Hickson.) sp()t of light) an d not an 



image, is seen. No image can be seen unless the micro- 

 scope be focussed for the centre of the cone ; and here there 

 are no structures capable of receiving it and transmitting 

 corresponding waves of change to the " brain." 



But what, it may be asked, can be the purpose of an 

 eye-structure which gives, not an image, but merely a spot 

 of light ? The answer to this question can only be found 

 when it is remembered that there are thousands of these 

 facets and cones giving thousands of spots of light. The 

 somewhat divergent cones and facets of the insect's eye 

 (Fig. 37) embrace, as a whole, an extended field of vision ; 

 each has its special point in that field; and each conveys 

 to the nerve-rod which lies beneath it a stimulation in 

 accordance with the brightness, or intensity, or quality of 

 that special point of the field to which it is directed. The 

 external field of vision is thus reproduced in miniature 

 mosaic at the points of the crystalline cones thus there is 

 produced by the juxtaposition of contiguous points a stippled 

 image. And it must be remembered that, even in human 

 vision, the stimulation is not that of a continuum, but is 



