Eyes of MoUuscs and Arthropods. 697 



at whicli tliey cntered, no more rays of lig-ht g-ainiug access 

 to the rhabdom than if no crystalline cone was presenti 



If we make the most g-enerous allowance possible for the theory 

 of mosaic vision, so celebrated on account of its wonderful acuteness 

 of conception. we are at best provided with organs that can only dis- 

 tinguish between light and darkness. But Exner, while admitting 

 that there is probably no pereeption of form by the compound eye, 

 considers that it is an organ very well adapted to perceive motion. We 

 are not prepared to discuss to any extent what the conditions necessary 

 for a motion-perceiving eye must be. It seems certain, however, that 

 any organ perceiving simply light gradations could never distinguish 

 motions of objects , except when they affected to a sensible degree 

 the amount of light; therefore, in that case, there could be no 

 pereeption of motion, but only a sensibility to the changes in the 

 amount of light. It seems to be equally certain that to perceive 

 motions of objects, it must be necessary for the light impres- 

 sions produced by these objects to have some definitio'n; 

 biit this is also just the condition essential to the pereeption of objects! 

 It is extremely probable that the image of an object in motion is more 

 irritant than that of one which is at rest. This is intelligible on the 

 ground that all other sensations, either of touch, smeli, hearing, or of 

 slight electric shocks , are much more excitant when interrupted, than 

 when continuous. We may consider that a moving object causes an 

 Image to be felt successively on various parts of the percipient surface, 

 causinga Vibration of Images, soto speak; in an ear, forinstance, 

 sound produces a succession of impressions upou the sa me Clements ; 

 in an eye the moving object produces a succession of impressions upon 

 different Clements. It has not been shown how the compound eye is 

 especially adapted for seeing objects in motion; since having the 

 percipient Clements either in a continuous layer, like that of the Verte- 

 brate eye, or isolated, as in Arthropods, cannot affect the principle of 

 the action. 



In conclusion, we may say that of two objects of equal luminosity, 

 other conditions being likewise equal, the one in motion would produce 

 a greater nervous Irritation than the one at rest, simply because 

 it would cause a succession of interrupted impressions; there- 

 fore the animai would be more likely to see the moving object than the 

 one at rest. The transmission of these impressions from one retinidium 

 to another would be as graduai in the compound eye as in that of a 

 Vertebrate, since, (1) the image of any object is, in most cases, not 



