394 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



condition is reversed, and it is in the horizontal meri- 

 dian that there is no convergence. 



The meridian of less curvature, then, is found, 

 and the difference between its curvature and that of 

 the greater determined. The difference indicates 

 the focal length of the lens required. A cylindrical 

 lens is then used and so placed that its convergence 

 added to that of the smaller curvature will make 

 the focal length of that meridian coincide with the 

 other. 



The astigmatism that has been described is regular 

 astigmatism. Irregular astigmatism consists of irregu- 

 larities of curvature in the same meridian. 



Aberrations of the eye, chromatic and 

 spherical. These aberrations, the causes of which 

 have been already described (chap, xxvii.), are not ab- 

 sent altogether from the eye, but their correction is pro- 

 vided for in very remarkable ways. Spherical aberra- 

 tion is met by the power of the iris to contract and 

 shut off outside rays, acting precisely as the diaphragm 

 in the camera obscura ; the refractive power of the 

 lens is less at the circumference than at the centre; 

 and the cornea is, owing to its form, less refractive at 

 the circumference than nearer to the optic axis ; 

 by such means, therefore, there is less refraction of the 

 outer rays. The aberration of colour is slight. Yet 

 it has been determined that the foci for red and violet 

 rays do not absolutely coincide in the eye, but that 

 there is an interval of about \ mm. The focus for 

 red rays is farther back than that for violet. The 

 power of accommodation is, therefore, more called into 

 play for red than for violet rays, and thus red objects 

 appear nearer to the eye than violet, though both be in 

 the same plane. Yet the amount of aberration is so 

 small that it is usually ignored. Its smallness is, doubt- 

 less, due to the different densities of the lens, already re- 

 ferred to, and to the different curvatures of the lens, 



