456 THE OUTGO OF ENERGY 



CALCULATION OF THE SITUATION AND SIZE OF 

 DIOPTRIC IMAGES 



Draw perpendiculars through the optical axis 

 of System C at the following points : the anterior 

 principal focus, </>i (7, the first principal point, 

 Si C, the second principal point, H z C, and the 

 posterior principal focus, </> 2 (retina). Mark on 

 the optical axis the first and second nodal points, 

 KI C and K, C. 



The following facts should be borne in mind : 



1. Every ray which in the first medium is 

 directed to the first nodal point appears in the 

 last medium to come from the second nodal 

 point and is parallel to its original direction. 



2. The point at which the ray cuts the second 

 principal surface is the same distance from the 

 optical axis as the point at which the ray cuts 

 the first principal surface ; between the prin- 

 cipal surfaces the ray is parallel to the optical 

 axis. 3. All rays parallel in the first medium 

 unite in one point in the second or posterior focal 

 surface (the plane passing through the posterior 

 principal focus vertical to the optical axis) ; if 



almost literally, the valuable works of Bonders (Accommodation 

 and Refraction of the Eye, New Sydenham Society, London, 

 1864) and Helmholtz (Handbuch der physiologischen Optik, 

 2te Auflage, 1896). 



