1872.] Br. R. J. Lee on the Sense of Sight in Birds. 



351 



differently according as they alter the tension of the liquid freely exposed 

 to the air, or the tension of the liquid in contact with the glass, which is 

 not of the same value. 



With respect to Proposition III. there is no difficulty. A liquid of con- 

 siderable contractile force, such as pure water, produces no separation of 

 salt in a solution of less contractile force. This explains a number of cases 

 described in a note by one of us submitted to the Society in July last*, in 

 which solutions exposed for hours together to heavy rain did not crystal- 

 lize, unless the rain brought down a speck of soot or some unclean body 

 that lowered the surface-tension of the solution. Indeed we know of no 

 liquid of superior tensile force to that of the solution, and not acting che- 

 mically upon it, that has any influence in producing crystallization. 



Proposition IV. also agrees with the phenomena. A glass rod or other 

 solid, more or less smeared with a film of a liquid of low tension, when 

 brought into contact with the solution determines crystallization by lower- 

 ing the surface-tension. Such, then, is the function of a nucleus with re- 

 spect to supersaturated saline solutions. If the solid be made chemically 

 clean, it may be plunged into the solution without altering its tension, and 

 hence there is no separation of salt. And here it may be remarked that 

 such a case is possible that a crystal of the salt itself may be brought into 

 contact with the solution without disturbing its tension, and hence be in- 

 active. It has never been pretended that a crystal of the salt is not a good 

 nucleus for a supersaturated solution of its own kind ; all that has been 

 stated by one of us is that, under special conditions, such a crystal may be 

 lowered into the solution without acting as a nucleus. 



III. " Remarks ou the Sense of Sight in Birds, accompanied by a 

 description of the Eye, and particularly of the Ciliary Muscle, in 

 three species of the Order Rapaces" By Robert James Lee, 

 M.A., M.D. Communicated by Robert Lee, M.D., F.R.S. 

 Received April 11, 1872. 



It is proposed in this communication to describe certain peculiarities in 

 the eye of the bird as compared with the eyes of other vertebrata ; and 

 further to examine to what extent those peculiarities enable us to explain 

 the remarkable powers of sight with which all species of birds are more or 

 less highly endowed. 



Those who study the habits and modes of existence of the lower animals, 

 find great interest in applying to various phenomena connected with them 

 the results of anatomical investigation, and in endeavouring to discover 

 such causes, or means adequate to produce such effects, as to render the 

 supposition of the existence of an indefinite property like instinct very 

 frequently unnecessary. 



This method it is my desire to apply in the explanation of those high 



* Supr&, p. 41 . 



2 c 2 



