REFRACTION IN THE EYE. 71 



dispersion of colors is not appreciated. 1 There is but a single 

 point in the retina, the f ovea centralis, where vision is abso- 

 lutely distinct ; and it is upon this point that images are made 

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



It is curious to note, however, that the refracting appa- 

 ratus is not exactly centred, a condition so essential to the 

 satisfactory performance of our most perfect optical instru- 

 ments. For example, in a compound microscope or a tele- 

 scope, the centres of the different lenses entering into the con- 

 struction 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 ob- 

 ject 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 ; and 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 de- 

 grees (Schuerman\ the vertical, from one to three degrees 

 (Mandelstamm)." * Of course, this want of exact centra- 

 tion of the optical apparatus, in normal eyes, does not prac- 

 tically 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 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 probable 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. 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 173. 



2 STELLWAG VON CARION, Treatise on the Diseases of the Eye, New York, 1868, 

 p. 669. 



