PecultarUles of Vifnn in Biriij. 473 



the whole, nearer the anterior extremity of the optic axis, than at any otlier time; that' 

 when by voluntary effort thofe mufcles are made to aft, the feveral circles of fcales will be 

 drawn back, and diniinill) the periphery of the eye at their refpedlve pofitlons in this lad; 

 fituation ; and that confeqiiently the internal capacity of the organ being rendered lefs, 

 the molt elaftic part, namely, the cornea, will yield to the preffure of the included humour, 

 and become more convex. This lie concludes mull render fmall obje£ls,near the animal, 

 very diflincl. 



On the relaxation of the mufcles, the original flatnefs of the cornea will be reftored by 

 the elafticity (as he affirms) of the fclerotica ; and he proceeds to make remarks on the ad- 

 vantage the animal fyftem derives from elafticity being oppofed to mufcular force, giving 

 nearly tlie fame inflances as were offered with regard to the elafticity of the cornea a few- 

 months before by iVIr. Home *. He does not explain in direcl terms the peculiar excellence 

 this imbricated fyftem of fcales may be imagined to poffefs, but thinks it particularly necef- 

 fary to birds, which, without it, would, as he fays, be continually expofed to daih themfelves 

 againft the trees of a thick foifeft. He conceives alfo, that the extraordinary alteration in 

 the focus of the eye of an eagle in almoll an inftant of time when it darts from the upper 

 air and feizes an objeft on the ground, and the purfuit and feizure of a gnat or fmall fly by 

 a fwallow, mult require the aid of this apparatus, which in them is very diftind. But with 

 regard to thcfe particulars, or final purpofes, fomewhat more of inveftigation feems to be 

 wanting. For it is certain, that no bird will dare to fly rapidly through a thick wood, but 

 either pafs leifurely from branch to branch, or foar above the tops of the trees when fpeed is 

 the obie£t of their aim. And however confiderably the bony circles may be (hewn to add 

 to the range of adjuftment for near objefts, it does not appear that any greater fuddennefs 

 of change, in the eagle and other birds of the fame fpecies, is wanted, than is u ithin the power 

 of animals not poffeffing that ftrudure; becaufe the adjuftment from remote to near ob- 

 iecVs is effetled in the human, and probably every other eye, in a time incomparably (horter 

 than that of an eagle's defcent upon his prey. Neither do fwallows purfue gnats or flies, 

 but feud through the air with at leaft twelve times the velocity of thofe fmall creatures, which 

 they probably catch by opening their mouths without the leall ufe of fight. 



The mention of fwallows, and the probability of winged animals ftriking themfelves 

 a'^ainft trees in their .light, brings to my rccoileclion feme particulars which do not feem 

 altogether foreign to the prefent fubje£l. In a certain ftate of the atmofphere, it is ufual 

 for fwallows to fly rapidly along near the face of a row of houfea, at the diftance of three or 

 four inches from the wall. Now I take it for granted, that they cannot poffibly diftinguilh the 

 houfes during their courfe, on account of the rapid angular motion -, and I doubt very much 

 whether they fee objctts before them with any precifion — not to mention the probability that 

 the eye itfelf would be immediately dried if the nictitating membrane were not almoft con- 

 (lantly upon it. With thefe rcfloftion.'!, on an occafion of this kind, it did not fcem furprifing 

 that many of thcfe birds came fo near my face, while obferving them frotn a window, that 

 they fcarccly feemed to have noticed me. But the moft remarkable circumftance was, that 

 they did, in fad turn alidc fo as to avoid ftriking mc j though in fomc inllances fo near that I 



' rhiloC Trwf. 1795, p. 2?; ortlm ]<"'">■''. P- !' '. ■ 

 Vot.I.— January 1 7y«. 3^ ' '• • " ''"*'' 



