ACTION OF LENSES ON LIGHT. 



743 



bleached chemically by the direct action of light, so that the pictures may be temporarily fixed 

 upon the retina, the comparison of the eye with the camera of the photographer becomes more 

 striking. In order that the passage of the rays of light through the media of the eye may be 

 rightly understood, we must know the following factors : (1) the refractive indices of all the 

 media ; (2) the form of the refractive surfaces ; (3) the distance of the various media from each 

 other and from the field of projection or retina. 



Action of a converging lens. We must know how a convex lens acts upon light. In a 

 convex lens we distinguish the centre of curvature, i.e., the centre of both spherical surfaces 

 (fig. 524, I, m, ?%). The line connecting both is called the chief axis ; the centre of this line is 

 the optical centre of the lens (o). All rays which pass through the optical eentre of the lens 

 pass through unbent, or without being refracted ; they are called the chief or principal rays 

 (n, n x ) The following are the laws regulating the action of a convex lens upon rays of light : 



S d 



Fig. 524. 

 Figures illustrating the action of lenses upon rays of light passing through them. 



1. Rays w T hich fall upon the lens, parallel with the principal axis (II, /, a), are so refracted 

 that they are collected on the other side of the lens, at a point called the focus or principal 

 focus (/). The distance of this point from the central point (o) of the lens, is called the focal 

 distance of the lens (/, o). The converse of this condition is evident, viz. , rays which diverge 

 from a focus and reach the lens, pass through it to the other side, parallel with the principal 

 axis, without again coming together. 



2. Rays of light proceeding from a source of light (IV, I) in the prolonged principal axis, but 

 beyond the focal point (/), again converge to a point on the other side of the lens. The follow- 

 ing cases may occur : (a) When the distance of the light from the lens is ecpual to twice the 

 focal distance, the focus or point of convergence lies at the same distance on the other side of 

 the lens, i.e., twice the focal distance. (&) If the luminous point be moved nearer to the 

 focus, then the focal point is moved farther away, (c) If the light is still farther from the lens 

 than twice the focal distance, then the focal point comes correspondingly near to the lens> 



3. Rays proceeding from a point of the chief axis (III, b) within the focal distance, pass out 

 at the other side less divergent, but do not come to a focus again. Conversely, rays which are 

 convergent, and pass through a collecting lens, have their focal point within the focal distance. 



4. If the luminous point (V, a) is placed in the secondary ray (a, b), the same laws obtain, 

 provided the angle formed by the secondary ray with the principal axis is small. 



Formation of images by convex lenses. After what has been stated, regarding the position 

 of the point of convergence of rays proceeding from a luminous point, the construction of the 

 image of any object by a convex lens is easily accomplished. This is done simply by projecting 

 images of the various parts of the object. Thus, evidently (in V), b is the focal point of the 

 object, a, while v is the focal point of the object I. The picture is inverted. Collecting lenses 

 form an inverted and real 'image (i.e., upon a screen) only of such objects as are placed beyond 

 the focal point of the lens. 



