NATURAL THEOLOGY. 31 



regulate itself, by inserting within it a machinery 

 which, by the artful use of the different expansion 

 of metals, preserves the equability of the motion 

 under all the various temperatures of heat and 

 cold in which the instrument may happen to be 

 placed. The ingenuity of this last contrivance 

 has been justly praised. Shall, therefore, a struc- 

 ture which difters from it chiefly by surpassing it, 

 be accounted no contrivance at all? or, if it be a 

 contrivance, that it is without a contriver ? 



But this, though much, is not the whole : by dif- 

 ferent species of animals the faculty we are de- 

 scribing is possessed in degrees suited to the dif- 

 ferent range of vision which their mode of life and 

 of procuring their food requires. Birds, for in- 

 stance, in general, procure their food by means of 

 their beak ; and, the distance between the eye and 

 the point of the beak being small, it becomes ne- 

 cessary that they should have the power of seeing 

 very near objects distinctly. On the other hand, 

 from being often elevated much above the ground, 

 living in the air, and moving through it with great 

 velocity, they require for their safety, as well as 

 for assisting them in descrying their prey, a power 

 of seeing at a great distance; a power of which, 

 in birds of rapine, surprising examples are given. 

 The fact accordingly is, that two peculiarities are 

 found in the eyes of birds, both tending to facili- 

 tate the change upon which the adjustment of the 

 eye to different distances depends. The one is a 

 bony, yet, in most species, aHexible rim or hoop. 



