6 Gee and Adamson, Dioptriemcters. 



from the margin at E of the aperture will have come from 

 the point P of the scale. In the diagram the dotted lines 

 correspond to a concave lens. Thejdeviation of the ra}- 

 produced by the lens will be the angle PEM. If the scale 

 reading corresponding to the point P be S scale divisions 



P M A P N 



Fig. 4. — Theory of Direct Method. 



(measured from the zero point M) the angular deviation 

 is given approximately by the ratio of the lengths : — 

 MP/EP = MP/CN = S scale divisions / x metres. 

 This angular deviation is nearly the same as the 

 deviation of a ray of light originally parallel to the axis of 



