72 SPECIAL SENSES. 



While we see certain objects with absolute distinctness in 

 a restricted field, the angle of vision is very wide, and rays of 

 light are admitted from an area equal nearly to the half of a 

 sphere. Such a provision is eminently well adapted to our 

 requirements. We direct the eyes to a particular point and 

 see a certain object distinctly, getting the advantage of an 

 image in the two eyes exactly at the points of distinct vision ; 

 the rays coming from without the area of distinct vision are 

 received upon different portions of the surface of the retina 

 and produce an impression more or less indistinct, not inter- 

 fering with the observation of the particular object to which 

 the attention is for the moment directed; but even while 

 looking intently at any object, the attention may be attracted 

 by another object of an unusual character, which might, for 

 example, convey an idea of danger, and the point of distinct 

 vision can be turned in its direction. Thus, while we see 

 distinctly but few objects at one time, the area of indistinct 

 vision is immense ; and our attention may be readily di- 

 rected to unexpected or unusual objects that may come with- 

 in any portion of the field of view. The small extent of the 

 area of distinct vision, especially for near objects, may be 

 readily appreciated if we watch a person attentively reading 

 a book, when the eyes will be seen to follow the lines from 

 one side of the page to the other with perfect regularity. 

 "When we consider that, in addition to these remarkable qual- 

 ities, which are never thought of in artificial optical instru- 

 ments, the eye may be accommodated at will, with the most 

 exquisite nicety, to vision at different distances, and that we 

 possess correct appreciation of form, etc., by the use of the 

 two eyes, it is evident that the organ of vision gains rather 

 than loses in comparison with the most perfect instruments 

 that ever have been, or probably ever will be constructed. 



Laws of Refraction, Dispersion, etc., bearing upon the 

 Physiology of Vision. 



In the present state of physiological science, we have lit- 



