150 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



The curvature of the cornea and the size of the pupil are 

 regulated by means of contractile fibres. The convexity of the 

 former can be increased, and the opening of the pupil diminished 

 to half its ordinary diameter. Strong chemical irritants of one 

 kind cause contraction, while others again make the cornea nearly 

 flat and widely extend the pupil. The lens is a true optical lens, 

 forming a perfect inverted image on the percipient elements, 

 namely the rods with their contained retinidia. If, when the eye 

 is on the stage of the microscope, a fine needle dipped in white paint 

 is inserted between the eye and the objective, a perfect inverted 

 image will be seen in the depths of the eye, and much larger and 

 more distant objects are represented with exactly the same precision 

 if presented in the right position. Thus Mr. Patten found that 

 by focussing upon the inner surface of the lens, an inverted image 

 of the moving tree-tops in front of the window could be seen. I 

 have tried the same experiment in various different ways, but with 

 only a moderate amount of success, because of imperfect manipulation 

 in isolating the eyes. I have tried it with the tentacular eyes and 

 with the mantle eyes, and with the shell eyes of Trigonia lamarckii. 



With the whole of these and the tentacular eyes of 

 Cerithium ebeninum, Brug., (See pi. v., fig. 6) I have been able 

 easily to obtain inverted images of different objects especially the 

 gas lamps with a glass shade under which I was operating, and of 

 the window of the room where my investigations are carried on. 

 But the slightest failure in the precautions necessary for success, 

 prevented these results. The eyes must be perfectly fresh and 

 uninjured, and mounted with the greatest care. The method 

 which succeeded best was to take the eye from the animal 

 immediately after death and drop it into osmic acid ; then 

 transferring it to absolute alcohol and clove oil, and mount it in 

 the ordinary way, taking care to avoid any undue pressure. Only 

 experience teaches the length of time required for exposure to the 

 re-agents, for the tissues are easily destroyed. 



I shall conclude the reference to Mr. Patten's observations by 

 the following important quotation. In answer to the question as 

 to where the image is formed, he says, "This may be easily 

 determined by following successively the layers of the eye as far 

 as the tapetum. First are seen the minute hexagonal ends of the 

 corneal cells, then the radiating and circular fibres of the pseudo- 

 cornea and outer surface of the lens, followed by the large 

 irregularly-shaped cells of the latter ; then the outer layer of 

 ganglionic cells above the perfectly regular but faint outline of 

 the rods, (which may be recognized by their resemblance to the 

 figures seen by viewing the prepared isolated retina from above 

 or below), — and lastly, the tapetum itself, from which issues the 

 red light from the pigment below. Just before reaching the 



