JeflVies.] 



254 [May 17, 



Dr. B. Joy Jeffries made a brief verbal communication on 

 the unity of design in the eyes of man and the lower animals. 



In the Harvard University Course of Lectures on the "Anatomy and 

 Physiology of Vision," which I am now delivering at Cambridge, I have 

 had occasion to especially study the unity of design in the visual 

 organs of man and other animals, and by means of my pictures and 

 diagrams I trust to make this evident to the Society. As every illu- 

 minated point in nature sends out rays of light in all directions, we 

 have only diverging rays, or those, which so far as the eye is con- 

 cerned, are practically parallel. There is needed a refractive me- 

 dium therefore, to bring such rays to a focus on the recipient surface, 

 where the stimulus of light finally causes nerve stimulation, sending 

 the sensation of light through an optic nerve to the brain or its rep- 

 resentative. We may take the human eye as the highest type, and 

 here we have refracting media ; namely, the convex cornea and double 

 convex crystalline lens behind it, by means of which diverging or 

 parallel rays of light are focussed on the recipient surface or retina, 

 which lines the interior of the eye-ball behind. Thus is formed what 

 exactly corresponds to a camera obscura. "We have refracting me- 

 dia in whose focus, or in the plane of whose focus, is placed a recipient 

 surface, which recipient surface or membrane or retina contains the 

 means or apparatus for causing the stimulus of light to give rise to a 

 nerve sensation to be transmitted to the brain. Now all eyes, no 

 matter what their external shape or appearance, if they answer these 

 postulates may of course be ranked together as constructed on one de- 

 sign or plan. If we follow down the series of vertebrates we shall 

 find these eyes all formed on this principle of the camera obscura. So 

 also in the simple or additional eyes of the rest of the animal kingdom, 

 we shall find a refractive apparatus, in the plane of whose focus is a 

 recipient membrane or retina. And this notwithstanding any differ- 

 ence in shape, size, or general appearance of the vertebrate eye, or 

 the simplicity of these so-called additional eyes of the insects. The 

 only other form of eye existing in the animal series is the com- 

 pound or facetted eye of the articulates. The common cornea of 

 this eye is divided up into a large number of distinct facets, (five 

 to thirty thousand), each one corresponding to a tube of pigment, so 

 to speak, in which is found the final termination of the optic nerve 

 fibre. The ray of light which enters through any one of the trans- 

 parent facets can only affect the optic nerve fibre termination corre- 



