IX CONDITIONS OF DISTINCT VISION 405 



correspond with the disposition of the luminous parts of 

 the object represented, and with the qualities of the light 

 which proceeds from them. 



That a biconvex lens is competent to produce a picture 

 of the external world on a properly arranged screen is a 

 fact of which every one can assure himself by simple 

 experiments. An ordinary magnifying glass is a trans- 

 parent body denser than the air, and convex on both 

 sides. If this lens be held at a certain distance from 

 a screen or wall in a dark room, and a lighted candle be 

 placed on the opposite side of it, it will be easy to adjust 

 the distances of candle, lens, and wall, in such a manner 

 that an image of the flame of the candle, upside down, 

 shall be thrown upon the wall. 



The spot on which the image is formed is called a focus. 

 If the candle be now brought nearer to the lens, the image 

 on the wall will enlarge, and grow blurred and dim, but 

 it may be restored to brightness and definition by moving 

 the lens further from the wall. But if, when the new 

 adjustment has taken place, the candle be moved away 

 from the lens, the image will again become confused, and 

 to restore its clearness, the lens will have to be brought 

 nearer the wall. 



Thus a convex lens forms a distinct picture of luminous 

 objects, but only at the focus on the side of the lens 

 opposite to the object ; and that focus is nearer when the 

 object is distant, and further off when it is near. 



Suppose, however, that, leaving the candle unmoved, 

 a lens with more convex surfaces is substituted for the 

 first, the image will be blurred, and the lens will have to 

 be moved nearer the wall to give it definition. If, on 

 the other hand, a lens with less convex surfaces is sub- 

 stituted for the first, it must be moved further from the 

 wall to attain the same end. 



In other words, other things being alike, the more con- 

 vex the lens the nearer its focus ; the less convex, the 

 further off its focus. 



