208 LABOR A TOR Y G UIDE IN PHYSIOL OGY. 



(1) Erect at n and p perpendiculars to the principal 

 axis. 



(2) Lay off, upon each, the two indices of refrac- 

 tion of the two media, measured from the origin 

 of each perpendicular, in the same linear units 

 used in measuring the radius. In the figure let 

 n c and p d represent the index of refraction of 

 the medium M, and n a and p b the index of re- 

 fraction of medium M'. The continuation of line 

 a d cuts the principal axis in the point f, the first 

 principal focus, while the line b c cuts it in the 

 point f, the second principal focus. The geo- 

 metrical figure shows the following important 

 properties of the dioptric system: 



I. The distance from the first principal focus to 

 the principal point equals the distance from the 

 second principal focus to the nodal point. 



(1) Mathematically expressed: pf=nf'. 



II. The ratio of the second focal distance (pf) to 

 the first (pf) is equal to the ratio of the index 

 of refraction of the second medium (M') to that 

 of the first (M).* 



(2) Mathematically expressed: pf: p{'=/j.'. //. 

 But p = nf; substitute this value in the second 

 equation, 



(3) .... nf: pi'p.: //; assume medium M to have 

 an index of refraction /*=!. 



(4) nf':pf'=l:/,'. 



(5) pf' = nf'X/j-'; or more concisely 



(5') p =//'n. (See p and n in Fig. 29. A.) 

 This derived property of the construction merits 

 a separate formulation. 



*Ref faction and Accommodation of the Eye. Landolt, p. 85. 



