VISION. 



rays from both meridians are focussed back of the retina, but at 

 two different points. 



(e) Simple myopic astigmatism is that form in which one merid- 

 ian refracts rays to a point in front of the retina, and the other 

 principal meridian fooisses rays upon the retina. 



(/) Compound hyperopic astigmatism is that form in which rays 

 passing through both principal meridians are brought to a focus at 

 different points in front of the retina. 



(g) Mixed astigmatism is that form in which one meridian fo- 



FIG. 4$. Described in text 



cusses rays in front of the retina, and the other focusses them back 

 of the retina. 



In simple hyperopic astigmatism the rays are focussed at 2 and 

 3 (see Fig. 45). 



In compound hyperopic astigmatism the rays are focussed at i 

 and 2. 



In simple myopic astigmatism the rays are focussed at 3 and 4. 



In compound myopic astigmatism the rays are focussed at 4 

 and 5. 



In mixed astigmatism the rays are focussed at 2 and 4. 



(/;) Presbyopia is loss of accommodative power due to sclerosing 

 of the crystalline lens. Although the process commences during 

 the first year of life, the lens does not lose enough of its elasticity 

 to interfere with near vision until about the age of fort}'. It is cor- 

 rected by placing before the eye a convex lens. Why ? Make a 

 diagram. 



CORRECTION OF REFRACTIVE DEFECTS. 



The Numbering of Lenses. Lenses are measured according to 

 their refractive power. A lens whose focal distance is one metre 



