148 



THE SENSE ORGANS 



[CH. 



Fig. 64. Diagram to explain 

 method of vision of the com- 

 pound eye. After Lubbock. 



diagram (fig. 64). If a number of transparent tubes be arranged 



side by side in front of a retinal 



screen, and separated from one 



another by a coating of black, the 



only light which will traverse each 



will be those rays parallel to the long 



axis of the tube (e.g. aa', bb', cc f ). 



All oblique rays (e.g. aw, ay, bx, bz) 



fall on the sides of the tube, and are 



absorbed by the black pigment. Now 



if the tubes be made very numerous, 



long and narrow, like the ommatidia 



of the compound eye, each separate 



unit would only perceive a very small portion of the total field of 



vision. The impression received through such an eye with 20,000 



ommatidia would be in the form of a landscape made up of 



20,000 little pieces, each accurate as to colour, distance and 



intensity. Anyone who has seen pictures constructed of mosaics 



will readily admit that such an impression would be an excellent 



representation of the scene, though not so perfect as the complete 



picture formed by the vertebrate eye. 



Now, let us suppose that a very small object, so small as only 

 to be represented on one of the 20,000 mosaic pieces, were suddenly 

 to move. The nerve-ending receiving the impression through the 

 corresponding ommatidium would at once respond to the move- 

 ment; the mosaic would be altered by a sudden alteration in one 

 of its pieces. Such a small movement might well pass unnoticed 

 by the vertebrate eye. But the compound eye would be quick 

 to register it, and the brain would act accordingly. If however 

 the change were made very slowly and steadily, it would probably 

 pass unnoticed. What we know of the power and method of 

 vision of the Dragonfly exactly agrees with this. The quiver of 

 an eyelid on the part of the collector is enough to frighten the 

 coveted prize away, though the stealthy approach of the collector's 

 whole bulk may not even be noticed. 



Huxley accepted the Mosaic Theory, with the important 

 addition that he considered the rhabdomes to be not merely 

 light-transmitters, but light-transmuters. That is to say, they 



