748 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



as the principal focus, F. There is another principal focus (F') in front of the 

 spherical surface viz. the point from which diverging incident rays will be 

 refracted into parallelism on passing the spherical surface, or, in other words, 

 the point at which parallel rays coming from the opposite direction will be 

 brought to a focus. The position of these two principal foci may be deter- 

 mined by the construction shown in Figure 214. Let CA C' represent a sec- 

 tion of a spherical refracting surface with the axis A N, the nodal point N, and 

 the principal point A. The problem is to find the foci of rays parallel to the 

 axis. Erect perpendiculars at A and N. Set off on each perpendicular dis- 

 tances No, Np, A o', Ap' proportionate to the rapidity of light in the two media 

 (e. g. 2 : 3). The points where the lines p' o and p o' prolonged will cut the 

 axis are the two principal foci F and F' i. e. the points at which parallel rays 

 coming from either direction are brought to a focus after passing the spherical 

 refracting surface. If the rays are not parallel, but diverging i. e. coming 

 from an object at a finite distance the point where the rays will be brought to 

 a focus, or, in other words, the point where the optical image of the luminous 

 object will be formed, may be determined by a construction which combines 

 any two of the three rays whose course is given in the manner above described. 

 Thus in Figure 215 let A JVbe the axis, and F and F' the principal foci of 



C \ 



FIG. 215. Diagram to show method of finding conjugate foci. 



the spherical refracting surface CA C', with a nodal point at N. Let B be 

 the origin of a pencil of rays the focus of which is to be determined. Draw 

 the line B C representing the course of an incident ray parallel to the axis. 

 This ray will necessarily be refracted through the focus F, its course being 

 represented by the line C F and its prolongation. Similarly, the incident ray 

 passing through the focus F' and striking the spherical surface at C' will, after 

 refraction, be parallel to the axis i. e. it will have the direction C' b. The 

 principal ray of the pencil will of course pass through the spherical surface and 

 the nodal point N without change of direction. These three rays will come 

 together at the same point 6, the position of which may be determined by con- 

 structing the course of any two of the three. The points B and 6 are called 

 conjugate foci, and are related to each other in such a way that an optical image 

 is formed at one point of a luminous object situated at the other. When the 

 rays of light pass through several refracting surfaces in succession their course 

 may be determined by separate calculations for each surface, a process which 

 is much simplified when the surfaces are " centred " i. e. have their centres 

 of curvature lying in the same axis, as is approximately the case in the eye. 



Refracting Media of the Eye. Rays of light in passing through the eye 

 penetrate seven different media and are refracted at seven surfaces. The media 



