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  |      "      |     9  |1 & 8 | Rigel.              |
| 9.| d    "     |      "      |    10  |2 & 8 | Red and white.      |
|10.| th   "     |      "      |        |      |Septuple.            |
|11.| l    "     |      "      |     5  |      |White and violet.    |
|12.| s    "     |      " A, B.|    11  |4 & 10|Octuple.             |
|13.| Castor     |      69     |   5-1/2|2 & 3 |White.               |
|14.| Pollux     |      69     |        |Triple|Orange, gray, lilac. |
|15.| g Virginis |      70     |     5  |3 & 3 |Both yellow.         |
------------------------------------------------------------

--------
]

When g Virginis was observed in 1718 by Bradley, the component parts were 7” asunder.  He incidentally remarked in his note-book that the line of their connection was parallel to the line of the two stars Spica, or a and d Virginis.  By 1840 they were not more than 1” apart, and the line of their connection greatly changed.  The appearance of the star is given in Fig. 75 (15), commencing at the left, for the years 1837 ’38 ’39 ’40 ’45 ’50 ’60 and ’79. also a conjectural [Page 213] orbit, placed obliquely, and the position of the stars at the times mentioned, commencing at the top.  The time of its complete revolution is one hundred and fifty years.