DEFECTS OF DIOPTRIC MEDIA. 573 



correcting effect on the chromatisra of the eye. Further correc- 

 tion takes place in the nerve centres which receive the sensation, 

 for just as we mentally rein vert the image, we fail to see the 

 color. At any rate the chromatic aberration is so slight that it 

 needs certain artifices to make it observable. 



Spherical aberration depends upon the fact that luminous rays, 

 on passing through a convex lens, strike the various parts of its 

 surface at different angles, and hence are differently refracted. 

 The rays striking the margin of the lens are more bent than those 

 passing through the centre, and hence the former come sooner to 

 a focus. Thus a luminous point gives rise to a diffused figure, 

 which is circular in perfectly centred dioptric systems, or stellate 

 in our eyes where the centring of the lenses is not absolutely 

 accurate. Spherical aberration causes us no inconvenience, as 

 the iris only allows the central rays to pass, upon which it can 

 produce no noticeable influence. 



Another optical defect in our eyes is astigmatism, depending 

 upon some irregularity of the curvature of the cornea, which may 

 be bent more horizontally than vertically, or vice versa. In either 

 of these cases the light in the vertical and horizontal planes will 

 be differently refracted, so that lines drawn in the two directions 

 will require different adjustments to see them distinctly. This 

 may be at once recognized if we gaze with one eye at the centre 

 from which many sharply-defined lines radiate, when only certain 

 ones can be seen distinctly, unless we move the eye or change its 

 accommodation. When the excessive curvature extends evenly 

 over the whole diameter of the cornea, it gives rise to what is 

 called regular astigmatism, and when the unevenness is localized 

 to one spot of the corneal surface it is called irregular astigmatism. 



The astigmatism which may be called physiological is not no- 

 ticed by the individual, but pathological astigmatism often occurs 

 and requires cylindrical glasses to correct it. 



Entoptic images are those which depend on the presence of some 

 opacity or difference in density in the transparent media of the 

 eye itself. They look like variously shaped specks moving over 

 the field of vision. They are only remarkable when we look at 

 an evenly colored object or through a pin-hole in a black card. 



