REFRACTION IN THE EYE. 691 



physical point of view. This statement, however, should not be understood 

 as implying that the arrangement of the parts is not such aa to adapt them 

 perfectly to their uses in connection with the proper appreciation of visual 

 impressions. By physical tests it can be demonstrated that the eye is not 

 entirely achromatic ; but in ordinary vision the dispersion of colors is not 

 appreciated. There is but a single point in the retina, the fovea centralis, 

 where vision is absolutely distinct ; and it is upon this point that images are 

 made to fall when the eye is directed toward any particular object. 



The refracting apparatus is not exactly centred, a condition so essential 

 to the satisfactory performance of perfect optical instruments. For example, 

 in a compound microscope or a telescope, the centres of the different lenses 

 entering into the construction of the instrument are all situated in a straight 

 line. Were the eye a perfect optical instrument, the line of vision would 

 coincide exactly with the axis of the cornea ; but this is not the case. The 

 visual line a line 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 mathe- 

 matical centre of the eye is observed both in the horizontal and in the verti- 

 cal planes. The horizontal deviation varies by two to eight degrees (Schuer- 

 man), and the vertical, by one to three degrees (Mandelstamm). Of course 

 this want of exact centring of the optical apparatus, in normal eyes, does 

 not practically affect distinct 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 mathemati- 

 cal 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 upon small objects. 



Although certain objects are seen with absolute distinctness only in a re- 

 stricted 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 emi- 

 nently 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 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 

 interfering with the observation of the particular object to which the atten- 

 tion is for the moment directed ; but even while looking intently at any ob- 

 ject, 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 but few objects are seen 

 distinctly at one time, the area of indistinct vision is very large ; and the at- 

 tention may readily be directed to unexpected or unusual objects that come 

 within any portion of the field of view. The small extent of the area of dis- 



