the text. As the book was not written for those who
possess the equipment of an observatory, with telescopes driven by
clockwork and provided with graduated circles, right ascensions and
declinations are not given. All of the telescopic phenomena described
are, however, represented in the maps. Star clusters are indicated by a
conventional symbol, and nebulae by a little white circle; while a small
cross serves to mark the places where notable new stars have appeared.
The relative magnitudes of the stars are approximately shown by the
dimensions of their symbols in the maps, the smaller stars being
represented by white dots and the larger by star-shaped figures.

In regard to binary stars, it should be remembered that, in many cases,
their distances and angles of position change so rapidly that any
statement concerning them remains valid only for a few years at the
most. There is also much confusion among the measurements announced by
different authorities. In general, the most recent measurements
obtainable in 1900 are given in the text, but the observer who wishes to
study close and rapid binaries will do well to revise his information
about them as frequently as possible. An excellent list of double stars
kept up to date, will be found in the annual Companion to the
Observatory, published in London.

In the lunar charts the plan of inserting the names of the principal
formations has been preferred to that usually followed, of indicating
them only by numbers, accompanied by a key list. Even in the most
detailed charts of the moon only a part of what is visible with
telescopes can be shown, and the representation, at best, must be merely
approximate. It is simply a question of what to include and what to
omit; and in the present case the probable needs of the amateur observer
have governed the selection--readiness and convenience of reference
being the chief aim.

It should, perhaps, be said here that the various chapters composing
this book--like those of "Astronomy with an Opera-glass"--were, in their
original form, with the single exception of Chapter IX, published in
Appletons' Popular Science Monthly. The author, it is needless to say,
was much gratified by the expressed wish of many readers that these
scattered papers should be revised and collected in a more permanent
form. As bearing upon the general subject of the book, a chapter has
been added, at the end, treating on the question of the existence of
plane