Vision is brought about by the images of objects being focussed on the retina. The focussing is effected by the refracting media of the eye, chiefly the lens arid the cornea. In order to accurate vision the image must fall in perfect focus on the retina. Variations in the focussing required by the greater or lesser distance of objects are brought about by the ciliary muscles acting on the lens. The condition of perfectly normal vision is designated Emmetropia. As life goes on the activity of the muscles and the elasticity of the lens lessen, and the power of rapid accommo dation diminishes. This condition is called Presbyopia. The defective power of accommodation is supplied by convex glasses. Other defects of vision occur when the image falls behind or in front of the retina, or is distorted.

In Hypermetropia the image falls behind the retina. This is usually due to abnormal shortness of the eyeball, but may also result from the refractive structures being deficient. Vision is good for distant objects but defective for near. In Myopia the image falls in front of tin retina. The axis of the globe is usually too long for the refractive structures (axial myopia), but sometimes the refractive stun tuns are too powerful (refractive myopia). Vision is defects far long distance and good at a short distance. In Astigmatism the image is distort from the refractive media, chiefly the cornea, varying in convexity at different meridians.

These various defects may lead to irritation and straining of the eye from the attempts to bring the image into focus.