The diagrams, it will be observed, are grouped under the seasons, and they indicate the positions of the constellations as they appear at 9 o'clock p.m. in mid-season.

To facilitate finding and observing the constellations, the student should face in the direction indicated in the text. This applies to all constellations excepting those near the zenith.

The four large plates are so arranged that the observer is supposed to be looking at the southern skies. By turning the plate about from left to right, the eastern, northern, and western skies are shown successively.

On many of the diagrams the position of nebulÆ is indicated. These are designated by the initial letter of the astronomer who catalogued them, preceded by his catalogue number, as for instance 8 M. signifies nebula number 8 in Messier's catalogue.

The magnitudes assigned to the stars in the diagrams are derived from the Harvard Photometry. When a star is midway between two magnitudes the numeral is underlined, thus 2, indicates a star of magnitude 2.5.

If a star's magnitude is between 1 and 1.5 it is regarded as a first-magnitude star. If it lies between 1.5 and 2 it is designated second magnitude.