195 



that of two faint and broad prismatic images, situated in a line per- 

 pendicular to that which joins the common coloured images, led the 

 author to the further discovery of the mode in which the fibres are 

 united laterally to each other, so as to resist separation, and form a 

 continuous spherical surface. By viewing a well -prepared lamina with 

 a microscope of high magnifying power, he observed that the fibres 

 are united by a series of teeth, locking into one another, exactly like 

 those of rack-work. The breadth and depth of each tooth are about 

 the fifth part of the breadth of the fibre itself, and all the adjacent sur- 

 faces are in perfect optical contact. This denticulated structure exists 

 in the lenses of every fish which the author examined. In that of the 

 cod, the number of teeth in each fibre was found to be 12,500 ; and 

 since the number of fibres in the whole lens is 5_,000,000, the total 

 number of teeth amount to 62,500,000,000. 



The same structure obtains universally, as far as the author has 

 examined it, in the lenses of birds ; but he has never met with it in 

 any of the Mammalia, not even in the Cetacea. It was found in two 

 species of lizards, and in the Ornithorhynchus. 



In the concluding part of the paper the author enters into some 

 details as to the doubly-refracting structure of the crystalline lens of 

 the cod and of other animals, in which several curious varieties are 

 observable with regard to the relative positions of the strata giving 

 positive or negative double refractions. In the prosecution of this 

 subject he was led to the observation of a series of very curious phe- 

 nomena, which he announces as the subject of a future communica- 

 tion to the Royal Society. 



A paper was also read, entitled, " On the present Situation of the 

 Magnetic Lines of Equal Variation, and their Changes on the Ter- 

 restrial Surface." By Peter Barlow, Esq., F.R.S. 



The author has undertaken the task of collecting and arranging 

 all the authentic information respecting magnetic variation which 

 has been recorded in the accounts of several recent voyages and 

 journeys of discovery. The inconvenience from the distortion and 

 interruptions of the lines of equal variation laid down on maps or 

 charts, induced him to trace them on a globe, where they can, of 

 course, be exhibited in their natural situation, and in regular conti- 

 nuity : and he has been careful to mark only such as are deduced 

 from actual observation. The examination of the lines thus laid down 

 shows them to be dependent on definite and general laws, and not 

 on local influences ; their inflexions and curvatures presenting systems 

 of great regularity, and being exempt from those abrupt and angular 

 configurations which such local disturbances might be expected to 

 produce : neither do they appear to be consistent with the hypothesis 

 of the action of a certain definite plurality of magnetic poles. 



The author next offers some observations on the progressive changes 

 which these lines undergo in their places and configurations, and 

 shows their agreement with the hypothesis of a revolution of the mag- 

 netic poles for each place round the poles of the earth ; each respec- 



