REFRACTION IN THE EYE. 735 



actly with the axis of the cornea; but this is not the case. The visual line (aline drawn 

 from an object to its image on the fovea centralis) deviates from the axis of the cornea, 

 in normal eyes, to the nasal side. The visual line, therefore, forms an angle with the 

 axis of the cornea. This is known as the angle alpha. This deviation of the visual 

 line from the mathematical centre of the eye is observed both in the horizontal and in 

 the vertical planes. " The horizontal deviation varies from two to eight degrees (Schuer- 

 man), the vertical, from one to three degrees (Mandtbtamm)." Of course, this want of 

 exact centration of the optical 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 various mathematical calculations connected with the physics of the eye. 



The field, or area of distinct vision, is quite restricted ; but, were it larger, it is proba- 

 ble that the mind would become confused with the extent and variety of the impressions, 

 and that we should be unable so easily to observe minute details and fix the attention 

 upon small objects. 



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 advan- 

 tage of an image in the two eyes exactly at the points of distinct vision ; the rays com- 

 ing 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 attract- 

 ed 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 directed to unexpected or unusual objects that may 

 come within 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 qualities, which are never thought of in artificial optical instruments, 

 the eye may be accommodated at will, with the most exquisite nicety, to vision at differ- 

 ent 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 have ever 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 little to do with the theory of 

 light, except as regards the modifications of luminous rays in passing through the re- 

 fracting media of the eye. It will be sufficient to state that nearly ail physicists of the 

 present day agree in accepting what is known as the theory of undulation, rejecting in 

 toto the emission-theory proposed by Newton. It is necessary to the theory of undula- 

 tion to assume that all space and all transparent bodies are permeated with what has been 

 called a luminiferous ether; and that light is propagated by a vibration or an undulation 

 of this hypothetical substance. This theory assimilates light to sound, in tin- mechanism 

 of its propagation; but, in sound, the waves are supposed to be longitudinal, or to fol- 

 low the line of propagation, while in light the particles are supposed to vibrato trans- 

 versely, or at right angles to the line of propagation. It must be remembered, however, 

 that the undulatory theory of sound is capable of positive demonstration, and that the 

 propagation of sound by waves can only take place through ponderable matter, the 

 vibrations of which can always be observed; while luminous vibrations involve the 

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