Aperture and Power required.—Opinions are divided as to the most suitable aperture and power for this work. Any telescope of from 4-to 10-inches aperture may be employed in it. A low power (30 to 50) and large field (50′ to 90´) eyepiece are imperative; and the instrument, to be really effective, should be mounted to facilitate sweeping either in a vertical or horizontal direction. A reflector on an alt-azimuth stand is a most convenient form for vertical sweeps. The defining-capacity of the telescope need not necessarily be perfect to be thoroughly serviceable, the purpose being to distinguish faint nebulous bodies, and not details of form. Far more will depend upon the observer’s aptitude and persistency than upon his instrumental means, which ought to be regarded as a mere adjunct to his powers and not a controlling influence in success, for the latter lies in himself. Very large instruments are not often used, because of their necessarily restricted fields. Moreover, a small instrument, apart from its advantage in this respect, is worked with greater facility and expedition. This is important, especially when the observer is to examine the region in the immediate neighbourhood of the Sun. He has then a very brief interval for the attainment of his purpose, and a small telescope must be used on account of its large field, its ready manipulation, and its general effectiveness on objects at low altitudes. The case is somewhat different when the search is to be conducted in regions far removed from the Sun’s place; for here the comets are in general faint, and there is time for the work to be deliberately and critically performed. Large instruments are to be recommended for these districts as capable of revealing fainter objects, though they are troublesome in several respects. They show large numbers of nebulæ, especially if the observer is exploring the region of Virgo, Coma Berenices, or Ursa Major; and he will have great difficulty in identifying them and in feeling his way with certainty. These complications are inseparable from the work, and, though chiefly affecting large apertures, should not always be shunned; for a telescope capable of displaying very faint nebulæ is also capable of showing faint comets. Many comets have eluded discovery by the inadequate reach of the instruments in the hands of comet-seekers; and the statement recently made that there are only about one hundred nebulæ liable to be mistaken for comets is not accurate, because comets in certain positions are of the last degree of faintness, and there is no identifying them from small nebulæ except by means of their motion.
Mr. Brooks says:—“Medium magnifying powers, with necessarily moderate-sized fields, are better than very low powers and large fields. While with the latter a large amount of the sky can be swept over in a given time, the work is not so well done, and a faint comet would be easily swept over and not seen. A small region, thoroughly worked, is far more likely to be successful. This gives a feeling of satisfaction with the work performed, even with negative results. In support of this I may remark that, during all the years I have engaged in comet-seeking, not a single comet has been discovered by another astronomer in a region of the heavens that I had just previously searched; so that I have never had occasion to feel that I had swept over a comet and missed seeing it. Aside from the obvious requirement of good eyesight, capable of detecting exceedingly faint objects, a good telescope of at least moderate aperture, and a familiarity attained by experience with the large number of nebulæ resembling telescopic comets, the comet-seeker, to be successful, must possess in a high degree the qualities of patience, perseverance, energy, and enthusiasm. I have the highest admiration for the man or woman who discovers a comet, because I know of the hard and thorough work which the success implies.”
Mr. Brooks’s experience and success in this branch give weight to his suggestions, and there can be little doubt that his commendation of moderate powers is fully justified. I believe he usually sweeps with a power of 40 (field of 1° 20′) on the 10-1/8-inch equatoreal of his observatory. Speaking for myself, I find powers of 32 (field 1-1/4°) and 40 (field 1°) perform very satisfactorily on my 10-inch With-Browning reflector, having frequently tried them on faint nebulæ and comets. Sometimes I employ a power of 60, field 50′; but for ordinary purposes this is too high. It is a good plan to sweep with a moderate power, say of 40, and to keep a higher magnifier at hand to examine any suspicious objects that may be picked up. With power 32 I often encounter forms, the real character of which is uncertain. In such cases I clamp the telescope and apply the power 60, which generally exhibits the objects as several minute stars grouped together, or possibly nebulæ, in which case I proceed to identify them. With lower magnifiers than 30 there must always be considerable danger of sweeping over faint comets. Some of these are only of the 10th, 11th, or 12th mag., and less than 1′ diameter, and must certainly elude detection unless adequate power is brought to bear upon them. Dr. Doberck mentioned in the L. A. S. Journal, vol. vi. p. 236, an instrument for comet-seeking, 3½ inches in aperture, power about 10, and field of 5°, which was bought in 1842 by the late Mr. Cooper at Markree. But though with such a telescope a very large portion of the firmament might be swept in one night, there would be serious disadvantages; for small faint comets would pass through the field unseen, and render the work abortive. The necessary conditions of the case go far to support the view that moderate powers and fields are best; for a search, to be thorough and satisfactory, must be done critically, and with a power capable of revealing the smallest specimens of comets.
Annual Rate of Discovery.—Arranging cometary discoveries during the century from 1782 to 1881 into periods of 20 years, and comparing the annual average with that during the last eight years, we get the following numbers:—
| Period. | Comets found. | Annual average. |
|---|---|---|
| 1782-1801 | 25 | 1·25 |
| 1802-1821 | 26 | 1·30 |
| 1822-1841 | 36 | 1·80 |
| 1842-1861 | 83 | 4·15 |
| 1862-1881 | 79 | 3·95 |
| 1882-1889 | 40 | 5·00 |
These discoveries seem to have been greatly accelerated about the year 1845. The yearly average between 1842 and 1881 was about 4; but between 1882 and 1889 it increased to 5, owing mainly to the diligence of Barnard and Brooks.
The months in which the largest number of cometary discoveries have been effected are July and August, the figures since 1782 being—
| Month. | Comets found. | Percentage. |
|---|---|---|
| January | 22 | 7·6 |
| February | 20 | 6·9 |
| March | 18 | 6·2 |
| April | 25 | 8·7 |
| May | 17 | 5·9 |
| June | 21 | 7·3 |
| July | 34 | 11·8 |
| August | 38 | 13·2 |
| September | 22 | 7·6 |
| October | 20 | 6·9 |
| November | 26 | 9·0 |
| December | 26 | 9·0 |
Of 289 comets discovered during the last 108 years, 123 belonged to the first six months, while no less than 166 belonged to the last half of the year.
Though comets are not confined to any special region of the heavens, there is no doubt that the vicinity of the Sun is the spot to which the comet-seeker should direct his chief attention. It is here where the majority of the discoveries have been made; and theoretically this should be so, seeing that the Sun is the controlling influence of the cometary flights, and that his position must be regarded as a sort of focus of their convergence and divergence. Hence the most likely spots are over the western horizon after sunset and the eastern horizon before sunrise. The twilight and zodiacal light, together with the mist at low altitudes, are impediments which are inseparable from this work; but they need not interfere to any serious extent if the observer is careful to make the best of his opportunities. But though special attention is recommended to the neighbourhood of the Sun, other regions should not be altogether neglected, for comets are occasionally found in nearly the opposite part of the heavens to the Sun’s place, as, for example, Zona’s Comet of November 1890. In order to save time, and to prevent troublesome references during the progress of sweeping, the brighter nebulæ should be marked upon a star-chart, so that, as they enter the field, they may be instantly identified.