OF FUNCTION; OR, HOW WE ACT. 25 



ing forces which maintained them in their places, 

 being in such exact equilibrium that this slight 

 motion of the atmosphere suffices to give the pre- 

 ponderance to the former. Of the chamois hunters 

 of the Alps he says 



From rock to rock, with giant bound, 

 High on their iron poles they pass; 

 **" Mute, lest the air, convulsed with sound, 

 Rend from above a frozen mass. 



This illustration, remote though it may seem, is 

 valuable as bringing clearly before the mind the 

 essential character of the process which constitutes 

 the animal function. For the stimulus in this case, 

 the aerial vibration, evidently produces the resulting 

 motion only by disturbing the equilibrium of the 

 counteracting forces. 



So, too, the photographic process is a true analogue 

 of the physical part of vision. To prepare a plate 

 for photographic purposes, it is only necessary to N 

 apply to it, in solution, chemical substances which 

 tend to undergo a change of composition, and the 

 equilibrium of which is so unstable as to be disturbed 

 by the rays of light. Thus prepared, the paper is 



