Page:Optics.djvu/79



77. There is another expression sometimes used, in which the distances are measured from the centre, (Fig. 70.)

Let $$EQ= q,\quad Eq=q',$$

78. It will be observed, that we have taken $$m$$ to represent the ratio of the sines of incidence and refraction in all cases, whether the passage of the light be into a denser or a rarer medium; if we chuse that $$m$$ should always represent the ratio of the sines of incidence and refraction out of the rarer into the denser, we must, in Cases 3 and 4, put $$\frac{1}{m}$$ for $$m$$.