DESIGxV IN THE ORGANIC WORLD. 425 



this wonderful organ possesses a power to which no optical instru- 

 ment of human construction can show the remotest parallelism, — 

 that of adjusting itself to differences of focal distance. Thus, if I 

 close one eye, and hold up my finger between my other eye and 

 the clock at the far end of the room, I cannot see both of them 

 distinctly at the same time, because, as they are at different dis- 

 tances from my eye, their pictures on my retina cannot both be 

 distinct. But, without moving either my head or my eye, I can 

 so " focus " my eye on either as to see it distinctly, the other 

 becoming hazy. This we all constantly do without the least 

 knowledge of the mechanism by which it is effected : and all that 

 the most careful and refined investigation has revealed to the 

 Physiologist, is that the focal adjustment is made by a change in 

 the curvature of the crystalline lens ; its curvature being increased 

 when the rays that fall upon it are more divergent, because pro- 

 ceeding from a nearer object; and being diminished when the 

 rays, proceeding from a more distant object, are less divergent ; — 

 so as in each case to bring them to a focus on the retina. This 

 change of curvature is produced, it is believed, by the action of 

 the ciliary muscle which surrounds the lens ; but hoiv that action 

 is called forth we do not know. Indeed, we are quite unconscious 

 that we are putting it into contraction, I simply determine, " I 

 " will look at the clock," or, " I will look at my finger," and my 

 eye adjusts itself accordingly. If, on the other hand, I were to 

 look with a telescope, first at a watch-face a few feet off, and then 

 at a church-clock at a distance, I should have to diminish the 

 distance between the object-glass and the eye-piece ; and I cannot 

 conceive of any optical mechanism by which the telescope could 

 be enabled to make this adjustment/^/- itself. That the eye should 

 be provided with such a mechanism, has always seemed to me a 

 most wonderful evidence of intelligent design ; and the importance 

 of this provision in our daily life is so great (as every one knows in 

 whom it is even partially deficient*), as to outweigh beyond all 



* While a person with good ordinary vision has a range of focal adjust- 

 ment from six or eight inches (ten inches being the ordinary " reading distance ") 

 to as many miles, that of a "short-sighted " person is limited to near objects, 

 and that of an elderly " long-sighted " person to distant objects. A complete 

 want of power to adjust the focus of the eyes is seldom met with ; but some- 

 times occurs as one of the odd local paralyses often left for a time by an attack 

 of diphtheria. 



