668 SPECIAL SENSES 



of the dioptric apparatus in normal eyes does not practically affect dis- 

 tinct vision; for when the eyes are directed toward any object, this 

 object is brought in the line of the visual axis ; but the angle alpha is 

 an important element to be taken into account in certain mathematical 

 calculations connected with the physics of the eye. . 



The area of distinct vision is quite restricted ; but were it larger, it is 

 probable that the mind would become confused by the extent and variety 

 of the impressions, and that it would not be so easy to observe minute 

 details and fix the attention on small objects. 



Although certain objects are seen with distinctness only in a 

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

 admitted from an area equal to nearly the half of a sphere. Such a 

 provision is eminently adapted to visual requirements. The eyes are 

 directed to a particular point and a certain object is seen distinctly, with 

 the advantage of an image in the two eyes, exactly at the points of dis- 

 tinct 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 interfering 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 charac- 

 ter, which might, for example, convey an idea of danger, and the point 

 of distinct vision can be turned in its direction. Thus, while but few 

 objects are seen distinctly at one time, the area of indistinct vision is 

 large ; and the attention may readily be directed to unexpected or unusual 

 objects that come within any part of the field of view. The small extent 

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

 appreciated in watching a person who is reading, when the eyes will be 

 seen to follow the lines from one side of the page to the other. 



Certain Laws of Refraction, Dispersion etc., bearing on the Physiology 

 of Vision. Physiologists have little to do with the theory of light, 

 except as regards the modifications of luminous rays in passing through 

 the refracting media of the eye. It will be sufficient to state that nearly 

 all physicists of the present day agree in accepting what is known as the 

 theory of undulation, rejecting the emission-theory proposed by Newton.. 

 It is necessary to the theory of undulation to assume that all space and 

 all transparent bodies are permeated with what has been called a lumi- 

 niferous ether ; and that light is propagated by vibration or undulation of 

 this hypothetical substance. This theory assimilates light to sound in 

 the mechanism of its propagation ; but in sound the waves are longitudi- 

 nal, following the line of propagation, while in light the particles vibrate 

 transversely, or at right angles to the line of propagation. 



