REFRACTION IN THE EYE 487 



and when the subject's eye is not accommodated 

 for either the light or the observer 's pupil, some 

 of the light reflected from the subject's retina will 

 reach the retina of the observer (Helmholtz). 



Influence of Angle between Light and Visual 

 Axis. 1. Draw a reduced eye with pupil of 

 four millimetres diameter as described above. 

 Draw to the margins of the pupil an illuminating 

 pencil of parallel rays that shall make with the 

 visual axis an angle of about 20. Draw the 

 course of these rays from the pupil to the retina 

 (seepage 457). On the opposite side of the visual 

 axis mark the nodal point of the observer's eye 

 in such a position that the observer's visual axis 

 shall make also an angle of about 20 with the 

 visual axis of the subject's eye. Draw rays from 

 this nodal point to the pupil, and thence to their 

 focus on the retina. 



The portion of the interior of the eye visible 

 to the observer will be that included between 

 the outermost rays of the two conical pencils, the 

 common base of which is the pupil. Note that 

 the apex of the cone is a short distance behind 

 the nodal point. The visible portion includes 

 therefore only a part of the anterior chamber, a 

 small portion of the lens, and a very small por- 

 tion of the vitreous. 1 



1 This matter is clearly presented by Dr. John Green in his 



