DIOPTRIC MECHANISM OF THE EYE 



283 



straight line FI is drawn from the anterior focus F parallel to the 

 incident ray AB, and the straight line ID from point / parallel 

 to the axis XX "; on joining point D y where the line cuts the 

 posterior focal plane O'O', with C, the line CD gives the direction 

 of the refracted ray. 



(cf) Construction of the Image of a given Object in a Centred 

 System. To determine the position on the image of point A of the 

 luminous object AB (Fig. 123), it is only necessary to know the 

 path of two rays starting from this point. If a first line AC is 

 drawn parallel to the axis, which cuts the second principal plane 

 VV at C, it must in consequence of refraction pass through 

 the posterior focal point F' 9 in the direction of CF'A. On draw- 

 ing a second line from A in the direction of the first nodal point 

 N t and a parallel to AN from the second nodal point N', this cuts 

 the line CF'A at A', the image of the object A. By the same 

 process the image of the object B coincides with B'. The whole 



V V 0' 



FIG. 123. Formation of the image of a point in a centred system. 



image A' I? of the object AB is an inverted real image, as can be 

 seen upon a projected plane. 



(h) Refraction of Light through Convex and Concave Lenses. 

 Lenses may be taken to represent a compound dioptric system, in 

 which the extreme media have the same refractive index, which 

 is lower than that of the central medium, and in which the two 

 refractive surfaces are at a less distance from each other than the 

 respective centres of curvature. 



In lenses the principal points coincide with the nodal points. 

 The optical centre of the lens is taken as that point of the optic 

 axis at which a ray of light is not deflected. The distance of the 

 optical centre from the two refractive surfaces of the lens is in 

 proportion to the radii of the surfaces. 



The refractive power of a lens is greater in proportion as its 

 focal distance is less, i.e. is inversely proportional to the focal 

 distance. A lens with a focal distance of 1 m. is usually taken as 

 the unit strength, and the refractive power of such a lens is called 

 a diopter (D.). Lenses of 2, 3, 4 . . . D. have, respectively, focal 

 distances of J, J, J . . . of a metre. 



Convex are distinguished from concave lenses. The former 

 have a positive focal distance. They cause the parallel rays 



