Boyal Medical Society of Edinlurgh. 295 



the hills on each side are seen what have been called the Parallel 

 Roads, — a series of shelves receding one above another, through 

 the whole extent of the glen. In most parts they are three in 

 number, in some parts only two : in one place five are distinctly 

 perceptible. Each shelf preserves a horizontal position through- 

 out the length of the glen. The second road is about thirty 

 yards lower than tlie first, and the third about sixty lower than 

 the second. In number, height, and position, they are similar 

 on the opposite sides of the glen. Their surface inclines out- 

 wards in a slope of about one foot in five, and their greatest 

 breadth is about twenty yards, but in many places much less 

 where the materials of ihe hills are hard. 



These shelves, which some have supposed to be artificial, Mr. 

 Dick shows, very satisfactorily, must have been produced by the 

 action of the surface of a vast lake, which must have filled the 

 valley, but undergone a series of successive subsidences, by the 

 bursting out of its water.'', corresponding to the number of 

 " roads" now visible. He has, he thinks, ascertained the point 

 in the glen through which the waters rushed when the lake sub- 

 sided to the second level. 



Mr, Dick supports his theory bv observations made on the 

 margins of deep lakes in the Highlands, and by an analogous 

 road or shelf, which surrounds a valley above the town of Su- 

 biaco. forty-six miles east from Rome, and which is known to 

 have been once on a level with the waters of the lake, by the 

 ruins of the baths of Nero, and of the aqueduct by which Ap- 

 pius Claudius conveyed water from this lake to Rome, though 

 the lake is now much lower. 



Dr. Brewster communicated experiments made by himself and 

 Dr. Gordon on the human eve, relating chiefly to the refractive 

 power of the aqueous, vitreous, and crystalline humours, and to 

 the polarizing structure of the diflferent parts of this organ. Con- 

 trary to the received opinion, the aqueous and vitreous humour 

 were found to have relractive powers greater than that of water, 

 that of the vitreous humour being the highest. The crystalline 

 lens exhibits a polarizing structure exactly the same as quartz, 

 or one set of doublv-refracting crystals, or the same as the mid- 

 dle coats of "the crystalline lens in fishes (Phil. Transac. Lond. 

 for 1816, p. oil.). The iris has the same structure, but the 

 cornea had an opposite structure, nearly the same as that of cal- 

 careous spar ; or the same as the outer and inner coats of the 

 crystalline lens in fishes. The tint polarized by the crystalline 

 of the human eye is a faint blue of the first order, 



A paper by Dr. Craigie was read, on the affinity between the 

 Persian and the Greek and Latin languages. 



A letter from T. Allan, Esq. gave a sketch of. the mineral 

 T 4 structure 



