REFRACTION IN THE EYE 443 



ing to Helmholtz, 1 the refractive index, n 2 , of the 

 aqueous humor is 1.3365 ; the ratio is |. 



Calculate Fj. and F z . The result is, 7^ = 23.266, 

 ^ = 31.095. 



2. The principal foci may also be approximately 

 found by construction. Erect at the principal 

 point and the nodal point 2 perpendiculars to the 

 optical axis. Set off on each perpendicular dis- 

 tances from the optical axis proportional to the 

 rapidity of light in the first and second medium. 

 The ratio in the case of the air and the aqueous 

 humor is 4 I 3. Mark therefore points 20 mm. 

 and 15 mm. from the axis. Draw a line from the 

 20 mm. point of the first perpendicular through 

 the 15 mm. point of the second, and produce the 

 line to the optical axis. Its intersection with 

 the optical axis, is the posterior principal focus. 

 Find in a -similar way the anterior principal 

 focus. Indicate upon the axis the cardinal points, 

 remembering that the construction drawing is to 

 be the natural size. 



Construction of Image. About 10 cm. in 

 front of the cornea draw an arrow, ij, which 

 shall intersect the optical axis at right angles. 



1 The figures for this and subsequent calculations under " Re- 

 fraction of the Eye," are those given by Helmholtz. They are 

 collected in a convenient Table on pages 461 and 462. 



a The centre of curvature is the nodal point of a system con- 

 sisting of a single spherical surface. 



