THE ORGANS OF SENSE. 



127 



the eye to diverge in such a way that they are focussed on the 

 retina. 



Astigmatism is usually due to unequal curvature of the cornea, the 

 commonest form showing a greater convexity in the vertical than in 

 the horizontal meridian. In other words, the cornea is not spherical, 

 but resembles the back of a spoon. Consequently the rays entering the 

 eye in the meridian of greater curvature are brought to a focus in front 

 of those which enter in the meridian of lesser curvature (fig. 39), and 

 as a result the eye cannot focus both bars of a cross simultaneously. 

 The defect is counteracted by the wearing of cylindrical lenses, that 

 is, lenses which resemble a vertical section of the superficial part of a 

 cylindrical glass rod. 



FIG. 



). Diagram to show the course of rays in an astigmatic eye. 

 From Starling's Principles of Physiology. 



(Waller.) 



The rays (from P) in the vertical meridian, vv, come to a focus sooner than those in the 

 horizontal meridian, M. 



The unit of measurement for degrees of refractive error and for the 

 strength of compensating lenses is the dioptre. This term indicates a 

 lens of such a strength that by it parallel rays are brought to a focus 

 at one metre distance. A 2 dioptre (2D) lens has a focal distance of 

 half a metre, a 3D lens of a third of a metre, a |D lens of two metres, 

 and so on. 



SPHERICAL AND CHROMATIC ABERRATION. 



The crystalline lens, like other lenses, has greater refractive power 

 at its periphery than towards its centre, and consequently rays which 

 pass through it near its margin are brought to a focus sooner than 

 those which pass through its centre, and give rise to a certain degree of 

 blurring of the image on the retina. The unequal refraction which 

 produces this result is known as spherical aberration, and it is corrected 

 in the eye (1) by the centre of the lens being denser and more highly 

 refractive than the peripheral portion, and (2) by the iris acting as a 

 diaphragm and cutting off the peripheral rays. 



By chromatic aberration is meant a fault common to all lenses and 

 shared by the crystalline lens, whereby each sector of the lens acts as a 



