SIGHT 231 



Presbyopia is defective vision due to the loss of power in 

 advanced years. The elasticity of the lens becomes less, and 

 the convexity cannot be increased for near vision. The ciliary 

 muscle may also be weaker and aid in the production of the 

 error. 



Astigmatism is a defect in the vision due to irregularity in the 

 globe of the eye whereby the diameter in one plane is greater 

 than in another. Thus, the cornea or the retina may be an 

 uneven surface, and the image focus definitely in one part and 

 falsely in another. In this condition vertical, variously oblique, 

 and horizontal lines are not seen with equal distinctness. Astig- 

 matism is corrected by the use of cylindrical or prismatic glasses, 

 which have to be accurately adapted to the needs of each case. 

 This error, if serious, usually accompanies other defects of vision. 



FIG. 16 



C' 



Hypermetropic eye. 



Diplopia is the condition which results from a want of har- 

 mony in the action of the eyes, so that the image of each eye is 

 perceived separately that is, two images are seen. Diplopia 

 is caused commonly by paralysis or spasm in one of the lateral 

 straight muscles, which results in an abnormal position of the 

 eyes. If the eyes are turned so that the axes of vision separate, 

 the condition is known as external strabismus, or squint; if the 

 axes are crossed, the result is internal strabismus, or cross-eye. 



When a pencil of rays falls on a spherical, refracting surface, 

 those at the periphery of the surface will be refracted more than 

 those which lie near the axis, and will come to a focus sooner. 

 This phenomenon is known as spherical aberration, and it 

 exists ordinarily as an imperfection in the eye, where it is 

 corrected largely by the greater refractive index of the centre 

 of the lens, and partly as well by the fact that the iris cuts off 

 the peripheral rays. 



