390 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



The knowledge of the physics of these conditions 

 indicates at once the means of correcting them. It is 

 evident that parallel rays ought to focus on the 

 retina. In the former condition (hypermetropia) they 

 come to a focus too late, i.e. behind the retina, in the 

 latter case (myopia) too soon. i.e. in front of the retina. 

 Obviously the interposition in front of the eye of a 

 converging lens just sufficient to bring forward the 

 focus to the retina, or of a diverging lens just sufficient 

 to displace the focus backwards to the retina, will 

 cure the conditions. This is what is done : the long- 

 sighted person gets a convex lens that adds to the 

 refraction of his eyes, and focusses parallel rays on his 

 retina; and the short-sighted person gets a concave 

 lens that diminishes the refraction of his eyes, and so 

 focusses the image on the retina. 



The focal distances of the lenses to be used can be 

 calculated by a formula. 



For hypermetropia the formula is 



111 



7 " ' i> " " d, 



where / = the focal distance of the convex glass 

 desired, D is the distance at which the object would be 

 held for distinct vision for a normal eve, and d the 



, / 



distance at which it is held for the long-sighted eye. 

 The normal distance D is usually taken as 10 inches. 

 Then 



1 1 1 



/" io " " d' 



Suppose the person requires to hold small type 

 printing he is desired to read, at 30 inches, d = 30. 

 Then 



1 _ 1 _l _ 2_ _ 1 

 / " ' 10 " " fcO " 30 ' -15 ' 



15 inches is the focal length of the desired con vex lens. 



