186 



SPECIAL PHYSIOLOGY 



surface of the separation of the two media being a spherical surface. 

 In the accompanying figure (Fig. 78) the spherical surface s's p s" 

 separates the medium m, whose refractive index is 1.000 from the 

 medium m 1 ', whose refractive index is 1.500. 



Note the following cardinal points of a simple dioptric system. 



The center of curvature of the spherical surface (n) in the nodal point. 



That radius which is the center of symmetry of the dioptric system 

 (e. g.j n-p) is called the principal axis of the system. In this axis 

 lie the first and second principal foci, f and f, respectively. The 

 point where the optical axis cuts the spherical surface p is called 

 the principal point. The plane tangent to the spherical surface at 

 this point is the principal plane. Planes perpendicular to the optical 

 axis at / and f are called the -first and second principal focal planes, 

 respectively. In the eye the second principal focal plane is the retina. 



Diagram to show the cardinal points of a simple dioptric system. 



1. Appliances and Materials. A white rabbit; support with 

 universal clamp holder and small cork-lined burette clamps; metre 

 stick or tape ; steel or ivory rule, with millimetres subdivided if pos- 

 sible; hand lens; fine dividers with needle points; bone forceps; 

 0.6 per cent. NaCl; camel's-hair pencil; absorbent cotton. 



1. Preparation. (1) Mathematical. (See Fig. 79.) We wish first 

 to locate the nodal point in the rabbit's eye. Represent the distance 

 from the retina to the nodal point by n ; the distance from the object 

 to the image by d ; the vertical dimension of the object by o ; the 

 same dimension of the image by i. From the similar right triangles 

 of the figure one may write: 



(1) 



(2) 



(3) 



o : i = d n : n 

 o n = i d in; 



n = 



id 



Under the conditions of the experiment i is so small compared 

 with o that it may be ignored in the denominator, and we may use 

 the equation: 



(4) 



id 



n = 

 o 



