6 THE OPTICAL DEFECTS OF THE EYE. 



been wearing glasses of 10 inch focus, or as they are sometimes called 

 No. 10 convex, or simply + 10 (plus 10). 



The method, however, that is usually adopted, depends upon a 

 property of convex lenses that will he more fully explained further on. 



If, for instance, you hold up a 10 inch convex lens at a distance of 

 10 inches from a white wall — the wall being about 20 feet from an 

 open window, opposite — there will appear, behind the lens, upon the 

 wall, an inverted, miniature picture of the window, and trees or build- 

 ings, &c., in front of the window. If the lens be held at a greater or 

 less distance from the wall than the focal length of the lens, the in- 

 verted picture will be indistinct. Measuring the distance therefore 

 that the lens must be held from the wall, to produce the sharpest pic- 

 ture, will give the focal length of the lens. 



Suppose, now, that we bring the lens to within, say 5 feet of the 

 window, and hold a sheet of white paper at the principal focal distance 

 behind the lens, viz., at ten inches, we will find a change in the in- 

 verted picture, there will still appear distant buildings, trees, &c. but 

 the sash of the window will be very indistinct. If, however, we move 

 the sheet of paper 12 inches from the lens — that is, two inches farther 

 from the lens, we will again see the image of the sash but scarcely any 

 trace of the buildings, trees, &c. This experiment is an illustration 

 of the fact that the nearer an object approaches the front of a convex 

 lens, the farther will be its image behind the lens ; thus, when an object 

 is 5 feet or rather 60 inches from the front of a 10 inch convex lens, 

 the inverted image is found to be 12 inches behind the lens ; when 30 

 inches, it will be 15 in. ; when 20, that is, double the length of the 

 focus, the image will be double the length of the focus behind the 

 lens; viz., 20 inches; when 15 inches, the image behind the lens will 

 be removed to 30 inches. As the object approaches the principal 

 focal distance of the lens the image recedes much more rapidly ; thus, 

 when at 12 inches, the image will be 60 inches ; when at 1 1, the image 

 will be 110 inches behind the lens. When however we bring the ob- 

 ject to within 10 inches of the lens — that is, at its principal focus, 

 there will be no image formed behind the lens, as the rays after pass- 

 ing the lens will be parallel. 



(I would strongly urge you, gentlemen, to perform all these experi- 

 ments for yourselves, as in that way only can you become familiar 

 with these important principles. These latter experiments can be 

 performed best in a dark room — taking for an object the flame of a 

 lamp or candle). 



