THE OPTICAL DEFECTS OF THE EYE. 7 



From the above we can easily understand the principle, 1st, that 

 the less divergent the rays of a pencil (that is, the nearer they ap- 

 proach parallel rays,) incident or falling upon a convex lens, the nearer 

 will the focus of the convergent pencil be to the principal focus of the 

 lens. 2nd. The moi'e divergent the incident pencil, the less convergent 

 (the more nearly parallel) will be the refracted pencil, and the more 

 distant will its focus be from the principal focus of the lens. 



Questions of the following nature very often arise in optics, viz., the 

 length of the principal focus of a convex lens being given, and the dis- 

 tance a certain object is in front of it ; — to find how far behind the lens 

 will be the inverted image of the object. Or to express it more tech- 

 nically, the length of the principal focus of a convex lens being given 

 and the length of the divergent incident pencil, to find the length of 

 the focus of convergent refracted pencil. Thus : Suppose you had the 

 following question : A 10 inch lens is 60 inches from an object; how 

 far behind the lens will be the inverted image ? 



This could be solved immediately, by actual trial, and measurement, 

 but this is not always practical. 



The rule given in some text books on optics is as follows : multiply 

 the length of the divergent incident pencil, that is, the distance the 

 object is from the lens, by the focal length of the lens, and divide by 

 the difference ; thus: 60x10=600, 60—10=50, 600 divided by 

 50=12 ; or |^=^"=12= the distance behind the lens. 



There is another property of convex lenses which I must not omit 

 to mention ; namely, what is called it magnifying power. 



When a convex lens is placed between the eye and an object, — 

 the object being at a less distance from the lens than its principle 

 focus, the object will appear enlarged or magnified. The shorter the 

 focus of the lens, tbe greater is its magnifying power. Thus, a 4 

 inch lens has a greater magnifying power than an 8 inch lens ; a 2 

 inch lens greater than a 4, and a 1 inch greater than a 2 ineh lens. 

 The 1 inch lens has, in fact, double the magnifying power of a 2 

 inch lens ; a 2, double that of 4 inch ; a 4 inch, double that of an 8 

 inch, &c. 



The " power " of a lens is therefore inversely proportional to its 

 focal length. For this reason a different form is used in expressing 

 the " power" or strength of a lens. A 1 inch lens is taken as unity. 



