PHYSIOLOGICAL OPTICS. 145 



to a focus at some point in the plane of the principal focus or the 

 PRINCIPAL FOCAL PLANE (P. P.P.). Rays which are appreciably 

 divergent when they reach the surface (that is which arise from 

 objects less than about ten metres away) have their foci behind 

 the principal focal plane. The focus moves farther back the nearer 

 the object is brought until finally a position of the object is reached 

 such that the rays from it after being refracted meet only at in- 

 finity, that is, they are parallel to one another. This position is 

 known as the first principal focus (P.P. Fig. 40). If the object is 

 brought nearer, the rays are merely rendered less divergent after 

 refraction and are not brought to a focus at all. 



Fig. 40 shows the way in which the position may be found of an 

 image formed by refraction at such a surface. From each limiting 

 point on the object, any two rays, the paths of which are known, are 

 followed until they meet. The place where they cross is the image 

 of the point from which they arise. 



Refraction by a Convex Lens. Rays, after they have entered 



the eye, have to pass through a convex lens, the crystalline lens, 



before they reach the light-sensitive cells. The refraction in this 



case is similar to that which we have just considered; like a simple 



convex surface, a convex lens converges rays which pass through it. 



The extent of the refraction depends on how sharply the surfaces of 



the lens are curved and on how much denser its substance is than 



the surrounding medium. The lens has a principal axis, which 



joins the centres of curvature of the two surfaces, a first and second 



principal focus, and a principal focal plane. The point where the 



principal axis cuts the surfaces is the principal point; rays which 



pass through here are not appreciably bent if the lens is a thin one. 



Experiment 54. Set in front of the opening of the lantern the 



ground glass screen, and the diaphragm with a vertical slit. 



Through a convex lens throw an image of the slit on the black 



wooden block. The slit is to serve as the "object". Now place 



against the lens a sheet of paper perforated with two holes, 



about 3 mm. in diameter, horizontally placed and less than the 



diameter of the lens apart. This stops all but two pencils of 



rays from the object. Note that the image does not disappear, 



but becomes less bright. Cover one hole and see the further 



dimming of the image. 



