The usual form of ice-crystals in clouds is a right hexagonal prism, which may be elongated as a needle or **foreshortened** like a thin plate.

As the line of centres becomes more ob.ique, the surface is seen more and more **foreshortened** and the brilliancy diminishes continuously.

As has been explained the mean distance of a group of stars can be readily determined from the parallactic motion, which, when not **foreshortened**, is approximately four times the parallax; but to obtain a complete knowledge of the distribution of stars it is necessary to know, not merely the mean parallax of the group, but also the frequency law, i.e.