263 



J, 



J? 



ON SOME EXPERIMENTS RELATING TO THE COM- 

 POUND EYES OF INSECTS. 



By E. J. Spitta, F.R.M.S., F.R.A.S. 



(Read December loth, 1905.) 



The compound eye of the insect appears outwardly to be an 

 aggregation of six-sided facets, each of which represents the 

 cornea of a simple eye called the " ommatidium." Each omma- 

 tidium produces an image of the object, so that the multitude 

 of images is merely the effect of numerous ommatidia. 



The subject, however, presents many difficulties, and is cer- 

 tainly very complicated. For the sake of brevity, and to make 

 what follows more intelligible, it will be as well to commence 

 by describing in a few words the structure of the human eye, 

 as typical of the form which one finds in many animals. It 

 is sufficient for the present purpose to say that this consists of 

 a front portion, or limiting membrane, called the " cornea," 

 behind which is a lens capable of alteration in form, so as to 

 cause a change of focal length to suit the ever-varying dis- 

 tance of different objects, and finally, at the back of the eye, 

 a screen upon which the image of the object is focussed, 

 termed the " retina." The cornea may be said to posse?s no 

 special refractive property — at any rate, none of importance in 

 comparison with the function of the lens. This we are led 

 to believe is true, because in cases of cataract— to cure which 

 the diseased lens has to be removed — all patients require some 

 sort of spectacles to enable them to see distinctly. In the 

 human subject the screen or retina is a most highly complicated 

 structure, and but little is definitely known of its various com- 

 ponents revealed by the microscope. For our present purpose no 

 detailed description is necessary of the different " layers," as they 

 are called, but it will suffice merely to mention the two leading 



