MODE OP VISION BY COMPOUND EYES. 163 



that the nerve which supplies them is but pmall ; that 

 the structure is not really analogous to that of a true 

 eye, and that the position would be very unsuitable, 

 one of them being actually situated on the stalk of 

 the compound eye. 



The question does not, however, seem to be by any 

 means clearly solved, and it must, I think, be admitted 

 that, with the exception of the anterior pair, if the 

 position does not seem suitable for true eyes, neither 

 is it that which one would expect in light-organs. 



On the ]\Iode of Vision by Means of 

 Compound Eyes. 



Johannes Mtiller, in his great work on the Physiology 

 of Vision,* was the first to give an intelligible explana- 

 tion of the manner in which insects see with their com- 

 pound eyes. According to his view (see Fig. 75), 

 those rays of light only which pass directly throngh 

 the crystalline cones, or are reflected from their sides, 

 can reach the corresponding nerve-fibre. The others 

 fall on and are ab.^orbed by the pigment which separates 

 the different facets. Hence eacii cone receives light 

 only from a very small portion of the field of vision, 

 and the rays so received are collected into one spot 

 of light. The larger and more convex, therefore, is the 

 eye, the wider will be its field of vision ; while the 

 smaller and more numerous are the facets, the more 

 distinct will the vision be. In fact, the picture per- 

 ceived by the insect will be a mosaic, in which the 

 number of points will correspond with the number of 

 fiacets. 



• "Zur vergleichenden Physiologic des Gesichtsinnes." 



