VISION. 5 



It is evidently, however, a mere conjecture that it is 

 by means of this membrane the eagle can look at 

 the sun*, for we do not see how it could be proved 

 by experiment, as it would be impossible to perceive 

 through it the direction of the pupil. Aristotle was 

 of opinion it was for the purpose of moistening the 

 eye ; but Aldrovand thinks we cannot well draw such 

 an inference, as our own eye is kept sufficiently moist 

 without such a provision f. 



Willughby describes the eye of the golden eagle as 

 having " a certain thick tunicle stretched forth from 

 below upwards, covering the globe in nictation. Two 

 eyelids, one above, the other beneath, although the 

 lower alone extending upwards is sufficient to cover 

 the whole eye. The region of the eyebrows is very 

 eminent, like that prominent part of houses called 

 the eaves, under which the eyes lie hid as it were 

 in a deep cavity. The eyes are of a green colour 

 (chlorops) of a fiery splendour, shining forth in a 

 pale blue. The pupil is of a deep black. It is very 

 admirable to observe what care nature hath taken, and 

 what provision she hath made for the conservation of 

 the eyes, than which there is no part in this animal 

 more excellent ; for not being content with one tegu- 

 ment, as is usual in other animals, she seemeth to 

 have framed four several lids or covers for them. 

 The membrane for nictation is the same thing, and 

 affords the same use to them that the eyelids do to 

 man. Besides which nature hath superadded two 

 other eyelids, arid oi these the lower so large that 

 they alone suffice to cover and preserve the eyej/' ! 



It was another remark of Aristotle, that the vision 

 of the osprey (Pandion haliaetus, SAVIGNY) is weak 

 on account of the shade which covers the eyes, 



* Blumenbach, by La\vrence and Coulson, p. 298. 

 t Ornithologia, i. 64. $ Ornithology, by Ray, p. 58. 



B3 



