Page:Optics.djvu/78

 :(3) A rarer medium with a concave surface, Fig. 68.
 * (4) A rarer medium with a convex surface, Fig. 69.

In all these figures $$QR$$ is the incident ray cutting the axis $$AE$$ in $$Q$$; $$RS$$ the refracted ray cutting $$AE$$ in $$q$$. $$E$$ is the centre of the surface.

76. It is not possible to obtain a general, simple, algebraical expression from this, but if merely the ultimate value of $$Aq$$ be required, that is, the limit of its value when the angle $$RQA$$ is diminished sine limite, we may put $$AQ$$, for $$RQ$$, $$Aq$$ for $$Rq$$ ; we have then