VOL. LXXTI.] PHILOSOPHICAL TRANSACTIONS. 323 



the divided edge near the fore part of the shell. The large eyes are at a great 

 distance from each other; but the small ones are close together. It will 

 appear hereafter, that the large eyes are made up of a great number of 

 small, transparent, amber-like cones, and that the small ones are composed 

 of one such cone only; so that they may be divided into eyes with many 

 cones, and eyes with a single cone. The large eyes, or those with many 

 cones, appear as 2 transparent spots about the size and nearly of the shape of a 

 kidney bean, the concave edges looking towards each other, and the convex 

 towards the edge of the shell. If they be examined attentively, we may discern 

 on their surface a number of small depressions, which point out the centre of 

 each cone. The small eyes, or those with a single cone, look like two small 

 transparent spots, not larger than a pin's head ; these, from their minuteness, 

 are easily overlooked. See tig. 8, pi. 5, where aa show the large eyes, and bb 

 the small ones. 



The appearances now described may be seen on the external surface of the 

 shell with the naked eye; but in order to proceed to a further investigation of 

 the subject, the cornea must be removed from the shell, and applied to a single 

 microscope with a very strong light. The internal surface of the large eyes, 

 examined with the microscope, is found to be thick set with a great number 

 (about 1000) of small, transparent cones, of an amber colour, the bases of 

 which stand downward, and their points upwards next the eye of the observer. 

 The cones in general have an oblique direction, except some in the middle of the 

 cornea, about 30 in number, the direction of which is perpendicular. The 

 centre of every cone being the most transparent part, and that through which 

 the light passes; on that account the perpendicular or central cones always appear 

 beautifully illuminated at their points. They are all so disposed, as that a certain 

 number of them receive the light from whatever point it may issue, and transmit 

 it to the immediate organ of sight, which we may reasonably suppose is placed 

 underneath them ; but this last circumstance can only be determined in a recent 

 subject, which Mr. A. has never been so lucky as to see. The cones are not all of 

 the same length ; those on the edges of the cornea are the longest, whence they 

 gradually diminish as they approach the centre, where they are not above half 

 the length of those on the edges : see fig. 9. 



As these cones so easily transmit the light through their substance, when 

 Mr. A. first examined them, he thought they were tubes; but having afterwards 

 viewed them broken in different directions, he was convinced they are solid 

 transparent bodies. If they be viewed with a deep magnifier, every cone appears 

 divided transversely by 2 or 3 internal septa or partitions. This appearance is 

 owing to the cones themselves being made up of several cones, one within 

 another, the septa or partitions being nothing more than the apices or points of 

 the external cones; but this will be further explained by considering that the 



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