'98 



PHYSIOLOGY. 



Astigmatism is a defect of refraction due to a want of symmetry 

 in the refracting media of the eye. The result of this is that the 

 rays of light passing through the lens are not brought to a focus at 

 the same point. This want of symmetry is usually in the cornea, 

 but may be in the lens. To remedy this defect we use a lens called 

 a cylinder to level up the curvature of one of the meridians of the 



Fig. 344. Different Kinds of Lenses. (GANOT.) 



A, Double convex. B, Plano-convex. C, Converging concavo-convex. D, 

 Double concave. E, Plano-concave. F, Diverging concavo-convex. C and F are 

 also called meniscus lenses. 



cornea to correspond to the curvature of the others. Cylinders have 

 no curvature in one axis, but more or less considerable curvature in 

 the opposite axis in correspondence with the degree of astigmatism 

 that has to be corrected. 



LENSES. Lenses are arranged according to their focal distance 

 in inches, and, as the unit was taken as one inch, all weaker lenses 



Fig. 345. Refraction of Rays in Regular Astigmatism. (BALL.) 



were expressed in fractions of an inch. However, Bonders made 

 the standard in lenses of a focal distance of one meter, and this unit 

 he called a dioptre. Thus the standard in a weak lens and the 

 stronger lens are multiples of these. Hence a lens of two dioptres 

 equals one of about twenty inches' focus. 



PURKINJE-SANSON'S IMAGES. If you place a lighted candle in 

 front of the eye of a person, three images of the flame are seen. 

 One, which is direct, small, brilliant, and comes from the anterior 



