308 PHYSIOLOGY CHAP. 



images. It can be corrected by cylindrical lenses of such a 

 strength that the asymmetry of the corneal curvature is com- 

 pensated, i.e. the refractive power of the horizontal meridian, or 

 of that meridian in which the refractive power is lowest, is 

 increased. When the correction is perfect, the degree of astig- 

 matism of the cylindrical correction -lens is equal to the degree 

 of astigmatism in the eye. 



(d) Physiological astigmatism depends not merely on the 

 asymmetry of the corneal curvature, but also on the fact that 

 the optical axis does not coincide with the visual axis, and that 

 the refractive surfaces of the different dioptric media are not 

 perfectly centred. The visual axis runs inwards and slightly 

 upwards, and strikes the fundus outside and a little below the 

 optic axis. In Fig. 128 (p. 288), of Helmholtz' schematic eye, 

 FF' is the optic axis, A V the visual axis. The angle formed by 

 the two axes is about 4 to 7 in the horizontal meridian, about 

 3-5 in the vertical. The rays that reach the eye along the visual 

 axis have therefore an oblique course. Owing to this fact a 

 cone of homocentric rays penetrating the eye in the horizontal 

 meridian becomes slightly astigmatic, because it is more refracted 

 than those which enter by the other meridians. It is therefore 

 true that the slight degree of astigmatism which thus arises is 

 over -compensated by the opposite astigmatism due to the 

 asymmetrical curvature of the cornea. 



Starting from the fact that the angle between the visual line 

 and the optic axis of the eye is usually about 5, Gullstrand 

 calculated the influence of the oblique incidence of the rays in 

 the visual axis of the schematic eye, and found that the focal 

 line does not exceed 0-03 mm., and the resulting degree of 

 astigmatism 0-1 D. These low figures sufficiently explain why 

 the oblique incidence of the line of vision causes no sensible 

 diminution of its acuity. 



The greater the angle formed by the incident rays with the 

 optical axis, that is, the more oblique their direction, so much the 

 longer will be the focal line, and so much the less clear the 

 corresponding image on the retina. The farther the retinal image 

 is from the centre at which the visual axis falls, and the more 

 eccentrically it lies on the retina, the more blurred will it be. 

 We shall deal more fully with these effects in examining indirect 

 vision, which depends on the peripheral region of the retina. 



The astigmatism due to oblique incidence of the rays may 

 also be due to imperfect centring of the curvature of the different 

 refractive media of the eye, because in this case again the rays 

 which fall parallel to the axis or perpendicular to the first surface 

 may fall obliquely on the other surfaces. Observations can be 

 found in ophthalmic literature to show that even in normal eyes 

 imperfect centring of the surface, as of the pupil, may be present, 



