648 THE SENSES. 



tions an object may be most easily perceived by indirect vision. It 

 often happens that in searching for a star of very small magnitude and 

 feeble light, it may be momentarily perceived by looking not directly 

 at it, but at a point in its immediate neighborhood, at a small angular 

 distance from its real position. The star is not seen distinctly under 

 these circumstances, because it is out of the line of direct vision. But 

 its light falls upon a part of the retina near the fovea centralis, the sen- 

 sibility of which is more acute than usual, owing to its continued 

 exposure only to the dark sky ; while the fovea itself, which has been 

 receiving in succession the images of particular stars, is comparatively 

 deficient in impressibility to light. When the visual axis is turned 

 directly upon the fainter star, for the purpose of getting a distinct 

 image, its light disappears ; and thus it can only be seen as an evanes- 

 cent object by indirect vision. 



If the eye be fixed immovably for too long a time upon the same 

 luminous object, the local diminution of retinal sensibility may amount 

 to fatigue ; and a persistence in its continuous application may produce 

 permanent injury of the visual organ. After steadily examining a 

 single object for even a short time, it becomes difficult to resist the 

 tendency to turn the sight in another direction by the automatic move- 

 ment of the muscles of the eyeball. Naturally, the eye never rests for 

 more than a few seconds upon an} r one point in the field of view ; but 

 is directed in succession at different objects, fixing each one in turn at 

 the point of distinct vision, and immediately passing to another more 

 or less remote. Thus the fatigue of the retina is avoided, since those 

 parts which at one instant have a stronger illumination, at the next 

 receive the impression of a shadow ; and no portion of the membrane 

 is .exposed sufficiently long to any single object to become insensible to 

 its grade of light or color. 



There is also reason to believe that the eye requires, for its safety, 

 the periodical suspension of all visual impressions which is obtainable 

 in sleep. It is not essentially different in this respect from other parts 

 of the nervous apparatus of animal life ; but the delicacy of its sensi- 

 bility, which is requisite for the due performance of its function, and the 

 complication of its structure, which includes so many parts adjusted to 

 each other with mathematical accuracy, indicate that it is one of the 

 organs most liable to derangement if deprived of its natural interval 

 of restoration and repose. 



Sense of Hearing. 



By the sense of hearing we receive the impressions of sound, and 

 appreciate their intensity, their tone or pitch, with all the variations 

 of higher or lower notes, as well as their quality, that is, the different 

 character of sounds of the same tone and intensity, but produced by 

 different methods, as by reeds, strings, or wind instruments, or by the 

 concussion of solid bodies. Our idea of time, or the succession of 

 events, seems also to be connected more especially with auditory sensa- 



