TOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 325 



each other, darting from every part of the field of view, towards a centre a little 

 exterior and superior to the axis of the eye. This centre corresponds to the in- 

 sertion of the optic nerve, and the appearance of lines is probably occasioned by 

 that motion of the retina which is produce4 by the sudden return of the circu- 

 lating fluid, into the veins accompanying the ramifications of the arteria centralis, 

 after having been detained by the pressure which is now intermitted. As such 

 an obstruction and such a re-admission must require particular circumstances, in 

 order to be effected in a sensible degree, it may naturally be supposed that this 

 experiment will not always easily succeed. 



Explanation of thejigures. — PI. 3, fig. 10, is a vertical section of the ox's eye, of the natural size, 

 A the cornea, covered by the tunica conjunctiva; bob the sclerotica, covered at bb by the tunica 

 albuginea, and tunica conjunctiva; dd the choroid, consisting of 2 laniinas; e k the circle of adlier- 

 ence of the choroid and sclerotica; fgfg the orbiculus ciliaris; hihk the uvea, its anterior surface 

 the iris: its posterior surface lined with pigraentum nigrum; ik the pupil; hlhl the ciliary pro- 

 cesses, covered with pigmentum nigrum ; mm the retina; n the aqueous humour; o the crystalline 

 lens; p tlie vitreous humour ; qrqr the zona ciliaris; rsrs the annulus mucosus. 



Fig. 11. The structure of the crystalline lens, as viewed in front. 



Fig. 12. A side view of the crystalline. 



XPII. Observations on a Current that often prevails to the IVestward of Sc'iUy; 

 Endangering the Safety of Ships that approach the British Channel. By J. 

 Rennell, Esq., F. R.'S. p. 182. 



It is a circumstance well known to seamen, that ships, in coming from the 

 Atlantic, and steering a course for the British Channel, in a parallel somewhat to 

 the south of the Scilly islands, often find themselves to the north of those islands: 

 or, in other words, in the mouth of the St. George's, or of the Bristol Channel. 

 This extraordinary error has passed for the efl^ects, either of bad steerage, bad 

 observations of latitude, or the indraught of the Bristol Channel; but none of 

 these account for it satisfactorily; because, admitting that at times there may be 

 an indraught, it cannot be supposed to extend to Scilly; and the case has hap- 

 pened in weather the most favourable for navigating, and for taking observations. 

 The consequences of this deviation from the intended track, have often been 

 fatal: particularly in the loss of the Nancy packet, in our own times; and that 

 of Sir Cloudesley Shovel, and others of his fleet, at the beginning of the present 

 century. 



I am however of opinion, says Mr. R., that these may be imputed to a spe- 

 cific cause; namely, a current: and I shall therefore endeavour to investigate 

 both that, and its effects; that seamen may be apprized of the times when they 

 are particularly to expect it, in anv considerable degree of strength; for then 

 only it is likely to occasion mischief; the current that prevails at ordinary times, 

 being probably too weak to produce an error in the reckoning, equal to the dif- 

 ference of parallel, between the south part of Scilly, and the track that a com- 



