352 



Dr. II. J. Lee on the S&nse of Sight in Birds. [May 16, 



and distant flights which are performed by certain species of birds in 

 search of food or in their migrations to different localities. 



For us it is difficult to form a clear conception of the power of sight 

 possessed by birds if we only use our own faculties in this respect as the 

 standard of comparison ; by which I mean to imply that the mind must 

 be prepared for the consideration of the phenomena referred to by observ- 

 ing in detail numerous important differences in the structure of the eye, 

 which combine to facilitate a conception of ideas otherwise beyond the 

 reasonable limits to which even imagination might extend. 



This field of inquiry will long engage the attention of the naturalist and 

 anatomist • indeed it may be said to be inexhaustible ; and I feel con- 

 siderable hesitation in offering a contribution insignificantly small to the 

 elucidation of a subject of such magnitude. 



We may acquire some idea of the sight of the bird by comparing the 

 dimensions of the eye with those of the brain or the optic lobes ; and by 

 arranging the measurements thus obtained, and referring them to some 

 fixed standard, we may estimate the relative and individual powers of 

 vision enjoyed by different species. In illustration of this we have an in- 

 stance, in the case of one of the birds which I propose to describe minutely 

 in this communication, in which the eye is actually considerably larger 

 than in the human species ; and we have a still more striking example, con- 

 sidering the size of the bird, in the Goura coronata. 



Again, if we regard the eye as an optical instrument, we may estimate 

 its efficiency by examining the internal structures on which the formation 

 and perception of the image depend, — such as the size and coefficient of 

 refraction of the lens, the extent and character of the retina, and particu- 

 larly those differences of minute structures which have relation to suscep- 

 tibility to light, by which the night-flying birds are distinguished from the 

 day-flyers. Nor does the inquiry into the effects of domestication upon 

 the sight appear less interesting. 



It is only to point out the various ways in which we may deal with this 

 subject that I have mentioned these different lines of research, and in order 

 that it may be understood that I have not overlooked their importance. 

 It is to one particular property of the eye that my own observations have 

 been chiefly directed, namely, the power of accommodation for distance ; 

 and I shall endeavour to show that in birds great range of vision depends 

 upon the development and character of the ciliary muscle, to which all are 

 agreed that the power of adjustment is to be attributed. 



It is chiefly, then, a comparison of the ciliary muscle in different birds 

 to which I invite attention, assuming the perfection of the sight to depend 

 on this power of accommodation, and that again on the character of the 

 muscle. Let me first mention the general opinion entertained by those 

 who are best acquainted with the habits of that class of birds which 

 astonish us by the rapidity and duration of their flights, namely the 

 pigeons, in regard to the means by which they accomplish them. In his 



