HISTORY OF 



force of his flying machine, he must be obliged 

 to increase its weight also. 



In all birds, except nocturnal ones, the head is 

 smaller, and bears less proportion to the body 

 than in quadrupeds, that it may more readily 

 divide the air in flying, and make way for the 

 body, so as to render its passage more easy. 

 Their eyes also are more flat and depressed than 

 in quadrupeds ; a circle of small plates of bones, 

 placed scalewise, under the outer coat of the or- 

 gan, encompasses the pupil on each, to strengthen 

 and defend it from injuries. Beside this, birds 

 have a kind of skin, called the nictitating mem- 

 brane, with which, like a veil, they can at pleasure 

 cover their eyes, though their eyelids continue 

 open. This membrane takes its rise from the 

 greater or more obtuse corner of the eye, and 

 serves to wipe, cleanse, and probably to moisten 

 its surface* The eyes, though they outwardly 

 appear but small, yet, separately, each almost 

 equals the brain ; whereas in man the brain is 

 more than twenty times larger than the orbit of 

 the eye. Nor is this organ in birds less adapted 

 for vision by a particular expansion of the optic 

 nerve, which renders the impressions of external 

 objects more vivid and distinct. 



From this conformation of the eye it follows, 

 that the sense of seeing in birds is infinitely 

 superior to that of other animals. Indeed, this 

 piercing sight seems necessary to the creature's 

 support and safety. Were this organ blunter, 

 from the rapidity of the bird's motion it would 

 be apt to strike against every object in its way ; 



