ould have if another sun were revolving about it. Even the
direction of the unseen body could always be indicated. In February,
1862, Alvan Clark, artist, poet, and maker of telescopes (which
requires even greater genius than to be both poet and artist),
discovered the companion of Sirius just in its predicted place. As a
matter of fact, one of Mr. Clark's sons saw it first; but their fame
is one. The time of revolution of this pair is fifty years. But one
companion does not meet the conditions of the movements. Here must
also be one or more planets too small or dark to be seen. The double
star x in the Great Bear (see Fig. 70) makes a revolution in
fifty-eight years.

Procyon moves in an orbit which requires the presence of a companion
star, but it has as yet eluded our search. Castor is a double star;
but a third star or planet, as yet undiscovered, is required to
account for its perturbations. Men who discovered Neptune by the
perturbations of Uranus are capable of judging the cause of the
perturbations of suns. We have spoken of [Page 212] the whole orbit
of the earth being invisible from the stars. The nearest star in our
northern hemisphere, 61 Cygni, is a telescopic double star; the
constituent parts of it are forty-five times as far from each other
as the earth is from the sun, yet it takes a large telescope to show
any distance between the stars.[*]

[Footnote *: _Telescopic Work._--Only such work will be laid out
here as can be done by small telescopes of from two to four inch
object-glasses. The numbers in Fig. 75 correspond to those of the
table.

--------------------------------------------------------------------
| | | |Dist. of|Magni-| |
|No.| Name. | Fig. | Parts. |tudes.| Remarks. |
|---|------------|-------------|--------|------|---------------------|
| 1.| e Lyrae | 72 | 1' 56" | |Quadruple. |
| 2.| z Lyrae | 72 | 44 |5 & 6 |Topaz and green. |
| 3.| b Cygni | 73 | 34-1/2|3 & 6 |Yellow and blue. |
| 4.| 61 Cygni | 73 | 20 |5 & 6 |Nearest star but one.|
| 5.| Mizar | 67 | 14 |3 & 4 |Both white. |
| 6.| Polaris | 67 | 18-1/2|2 & 9 |Test object of eye |
| | | | | | and glass. |
| 7.| r Orionis |Frontispiece.| 7 |5 & 8 |Yellow and blue. |
| 8.| b Orionis |