\magnification=\magstep1


$$
\eqalign{
X \quad &= \quad { {H} \over {\cos z} } \cr
        & \cr
        &= \quad { H \sec z }  \cr
}
$$

$$
 {\rm airmass\ X } \quad = \quad \sec z 
$$


$$
I \quad = \quad I_0 \thinspace e^{-f X}
$$

$$
m \quad = \quad m_0 + k X
$$

\bye
