474 Suppi/iJ Sixth Seiife in KuS. 



both heard and fit tlic aftion of the air to which thc-y gave motion. On more attcntivr 

 confideration, however, it appeared to nie, that this motion of the air is the very inP.rumcnt 

 which by its re-acflion may warn them of tlie prefcncc and polition of an obllacle tlic m- 

 ftant they approach it; which they may habitually and fpeeddy avoid. For, as the intelli- 

 gent blind are fald to know, by the motion of air and its echo, the (izc of rooms, the width of 

 ftrcets, and otlscr local attributes, which are commonly afccrtained by the fenfe they are depri- 

 ved of; and as boats move more (luggiflily in lliallow or contra£lcd channels ; and air itfclf 

 cannot be forced with rapidity through long tubes ; — in all which circumilanccs the re-a£lion 

 niufl be very confidirable; (o, on the other hand, it can fcarccly be doubted, that the object of 

 fuch re-a£lions, if capable of perception, would be fulTicienlly aware of their prefence or ab- 

 fence. I fuppofe tlie rc-ac^icn of the air daP.ied againft my face by the fwallow, and the noife I 

 heard, were as perceptible and as imprefTive to the bird as to myfelf ; and from this reafoninf 

 I am lefs difpofed to wonder at the refult of the cruel experiment performed in Italy a few- 

 years ago upon bats. As I have feen the notice only in a voluminous periodical publication 

 to which I cannot now turn, I mull fimply relate, that bats have been thought to polTefs a 

 fixth fenfe, becaufe, when the eyes of one of thofc animals were diflcfled out, it flew round 

 a room with the fame fpeed and precifion as before, avoiding the walls and obftacles 

 as readily as if ftill poffeffed of fight. If my conjeflures have any foundation, thefe 

 creatures did never ufe the eye to warn them of impediments to their flight, but had 

 conftantly attended to the re-a(flion of the fluid in which they moved ; which would be very 

 different in the vicinity of an obilacle compared with that in open fpace. From the re- 

 a£Vion of a direifl obftacle, they would turn to the fide where the prefTure was Icaft, or the 

 fpace moft open ; and if the obftacle were oblique, it may eafily be imagined that the le- 

 quifite deviation from their cnurfe would be equally obvious. 



To return to Mr. Smith's Memoir. He was led from the examination of the eyes of 

 birds to thofe of quadrupeds, and found by tearing and difiecling that the refti mufcles ter- 

 minate in the cornea; in which, and in his inferences, he entirely agrees with Mr. Home in 

 the paper before mentioned*. 



The Croonian Ledture, read before the Royal Society in November 179S, was written 

 by Everard Home, Efq. and contains a profecution of the enquiry refpeding vifion, of 

 which an abridgement has already been given. 



The explanation of the adjuflment of the eye by an affumed change in the radius of 

 the cornea being different from the theories before formed on that fubject, it was thought 

 light to put it to the teft of every experiment which might appear likely to refute or con- 

 firm the obfervations already made. The reader has feen that a perceptible variation in the 

 figure of this part of the organ of fight, was afcerlained by obferving its outline by means 

 of a microfcope. Another method fuggefted itfelf; namely, that if the convexity of the 

 cornea were increafed to a certain degree, it could be meifured by applying an achromatic 

 microfcope, with a divided eye-ghfs micrometer, to view the image in the virtual focus of its 

 furface. The firfl ftep in this experimental procefs was to afcertain, by experiments upon 

 convex mirrors, what difference of curvature could be decidedly obferved by an apparatus 

 of this kind. Two convex mirrors, one of ^%\hi of an inch focus, the other ^Si^hs, had their 



* Fbilor. TranT. or this Journal a> laft quoted. 



^ flat 



