572 William Patten 



return to its naturai position, after the coutractionofits 

 peripheral, cireular fibres has relaxed the tension upon 

 the centrai portìou. There is good evidence of the elasticity of this 

 membrane, for in certaìn reagents, as chromic acid, the muscles being 

 gradually deprived of their power, the tension of the septal membrane 

 acting upon the lens presses it almost fiat, and two great folds are pro- 

 duced in the periphery of the retina. But, by isolating the lens and sub- 

 sequently hardening it in chromic acid, its originai shape is perfectly 

 preserved. Muscular contractions which, caused by the Irritation of rea- 

 gents, produce such a great difiference in the shape of the lens, must be 

 able, in the living condition, to produce the extremely slight changes 

 necessary for focal adjustment ! This process may be produced then in 

 two ways ; by an increased convexity, causing a shorter focal distance, 

 and by the bodily movement of the lens itself, But it will be asked, is 

 this body really a lens? Does it actually form an Image, or concentrate 

 the rays of light at a certain point, and does this point fall within the 

 retinal layer which, morphologically, we mustconsider to be the essen- 

 tial and percipientone? 



These questions are pertinent, and the answer to them will show 

 whether the body in question is really entitled to be called a lens. and 

 whether the function of these eyes of Pecten is as perfect as their com- 

 plicated structure indicates. 



The following experiments will prove that ali the above questions 

 may be answered in the affirmative. The body in question is a true, 

 optic lens, it forms a perfect inverted image falling upon what must, 

 morphologically. be considered as the percipient Clements, namely, the 

 rods with their contained retini dia. In order to see the Images form- 

 ed by the lens, and determine their position, one of the large eyes, 

 containing little pigment, must be selected from the anterior or posterior 

 edge of the mantle : one may then easily look into the interior of the 

 eye, wheu it has been carefully removed from the shell. The eye must 

 then be fixed with the optic axis nearly vertical. but mth a slight incliu- 

 ation toward the window whence the light comes. 



The Images of white objects are most easily visible. I bave, 

 therefore, dipped a fine needle in white paiut, leaving a small globule 

 at the point. If the needle is now inserted between the eye and the ob- 

 jective, a perfect inverted image will beseen in the depths 

 of the eye; the globule at the end of the needle serves to orient the 

 image. That the image of a much larger and distant object will be 

 formed with exactly the sanie precision may be proved by holding the 



