148 



Royal Society : — 



explanation of the means by which accommodation for distance is 

 effected, if their researches have been attended with the observation 

 of any previously unknown facts connected with the subject, either 

 experimental or anatomical. It appears to me that as yet we have 

 not sufficient data to afford a perfectly satisfactory explanation of 

 that remarkable property possessed by the eye, partly on account 

 of the difficulty of ascertaining the exact functions of different struc- 

 tures, and particularly by reason of the very various conditions which 

 the same structures assume in various species of vertebrate animals. 

 The line of investigation which is pointed out in this communication 

 it is by no means certain will assist in the solution of the problem 

 of the means by which adjustment for distance is effected ; but I am 

 inclined to think that we have not yet exhausted all the resources 

 which careful anatomical inquiry places at our command, and that 

 when a sufficient number of details have been collected, the subject 

 will be in a more suitable state for the application of optical laws than 

 it is at present. 



Supplement, containing a description of the Eye in Rhea Ameri- 

 cana, Phcenicopterus antiquorum, and Aptenodytes Humboldtii : — 



In the American Ostrich the eye is large, and the structures con- 

 cerned in the adjustment for distance are well developed. In the 

 Ostrich (Struthio camelus) the observation was first made by Sir 

 P. Crampton of the existence of the ciliary muscle ; and as the views 

 of physiologists regarding the mechanical functions of the muscle in 

 the accommodation of sight were various, while numerous inquiries 



