Eyes of Molhiscs and Arthropods. 573 



fing'ers of the band at the level of the ocular, when an upright image 

 of the fiugers, or a part of the band, will be seen with perfect distinct- 

 uess, below the iuuer surface of the lens. An upright image is seen in 

 the latter case, because the inverted image, formed by the lens, is recti- 

 fied by the microseope. In the former experiment, both the nee dl e 

 and its inverted image are reversed by the microseope, so that the 

 relation between the two is retained. Any other object, when held in 

 the right position, may be seen with equal distinctness. For instancC; if 

 one of the numerous tentacles, supplied with sense hair papillae, be 

 placed over the pupil, its inverted image, with all the details of struc- 

 ture, will be seen below the surface. One may also paint several white 

 lines on the base of the objective, the Images of which will easily be 

 seen in the eye. That these Images are not formed by reflection from 

 the cornea, or snrfaces of the lens, is proved: first, by the fact that the 

 Images are inverted and are formed below the lens; and, secondly, by 

 comparing them with the reflected Images formed by the surface of the 

 lens, or cornea. For instance, by focusing upon the latter, or inner sur- 

 face of the lens, an inverted image of the moving tree tops in front 

 of the window, or the base of the latter will be seen. When seen with 

 the microseope, these images are inverted, andtherefore are formed 

 by the cornea and surface of the lens, as upright and 

 reflected images. Only apart of the image, that formed in the 

 centre of the eye, is perfect; the image of a large object, as the band 

 or fingers, about twelve inches from the eye, will be enlarged and curved 

 at the periphery. Only the end of the horizontally held finger will be 

 perfectly reproduced,whilethe basal part appears enlarged and beut ; but 

 a tentacle, just above and extending entirely across the pupil, will be 

 quite perfectly reproducedas a concave image. In trying this simple 

 experiment, the utmost care is necessary to bave a perfectly fresh and 

 uninjured eye, as the slightest pressure appears to be sufficient to pre- 

 vent the formation of any but reflex images. But where is the image 

 formed ? This may be easily determined by following successively the 

 layers of the eye as far as the tapetum. First, are seen the minute hexa- 

 gonal 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 regulär 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 



