Telescopic Work for Starlight Evenings. William F. Denning
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may be so arranged as to hold eyepieces, and are to be further utilized when making drawings at the telescope. I find it possible to obtain very steady views of celestial objects in this way. Everyone knows that during a critical observation it is as essential for the observer to be perfectly still as it is for the instrument to be free from vibration. A person who stands looking through a telescope feels a desire to ensure a convenient stability by catching hold of it. The impression is no doubt correctly conveyed to his mind that he may obtain a better view in this way; and so he would, were it not for the dancing of the image which instantly follows the handling of the instrument. For this reason it is absolutely necessary that no part of the observer touch the telescope while in use. He must ensure the desired steadiness, which is really a most important consideration, by other means; and an observer who provides for this contingency will have taken a useful step in the way of achieving delicate work.
Advantage of Equatoreals.—Those who employ equatoreal mounting and clock-work will manifestly command an advantage in tracing features on a planet or other object requiring critical scrutiny. Common stands, though often good make-shifts, require constant application on the part of the observer, when his undivided attention should be concentrated on the object. With an alt-azimuth stand nearly one half the observer’s time is occupied in keeping the object near the middle of the field. Though good views are obtainable, they are very fugitive. Just as the delicate features are being impressed on the retina they are lost in the ill-defined margin of the field, or from the necessity of suddenly shifting the object back. A succession of hurried views of this kind, during which the observer is frantically endeavouring to grasp details which only require a steady view to be well displayed, are often tantalizing and seldom satisfactory in their issues. This is especially the case when a single lens and high powers are used, and if the night is windy the difficulty is intensified. It is, therefore, evident that a clock-driven telescope possesses marked advantages in delicate work on faint objects, because the prolonged view better enables the eye to gather in the details which are all but lost in the elusive glimpses afforded by inferior means. Still we must not forget that rough appliances do not present an effectual barrier to success. The very finest definition comes only in momentary glimpses. The sharply-cut outlines of planetary configuration cannot steadily be held for long together. Only now and then the image acquires the distinctness of an engraving, when the air and the focus of eye and telescope severally combine to produce a perfect picture. Observers, therefore, whose instruments are simply, though perhaps substantially mounted in handy fashion, must profit by these moments of fine seeing, and, when drawing, will find it expedient to fill in, little by little, the delicate forms which reach the eye. This will take much time owing to the drawbacks alluded to, but the outcome will more than justify its expenditure, and the observer will gain patience and perseverance which will prove a useful experience in the future.
Lenses out of centre or misplaced are, like other defects, calculated to give rise to errors as numerous as they are various. But the most striking of these apparently belong to a period when telescopes were far less perfect and popular than at the present day. Indeed, it is surprising that so very few false or imaginary discoveries are announced when we consider the vast array of instruments that are now employed. It is true we occasionally hear that a comet has been discovered close to Jupiter, that several companions have been seen to Polaris, or that some other extraordinary “find” has been effected, but the age is dead when such announcements were accepted without suitable investigation. The satellite of Venus has long since ceased to exist. The active volcanoes on the Moon have become extinct. Even Vulcan will have to be set aside, and, like many another sensation which caused quite a furóre in its day, must soon be altogether expunged from the category of “suspects.”
Test-objects. Opticians sometimes advertise lists of objects—generally double stars—which may be seen with their instruments, but it does not appear to be sufficiently understood that the character of a telescope is dependent in a great degree upon the ability of the observer, who can either make or mar it, according to the skill he displays in its management. Some men will undoubtedly see more with 5 inches of aperture than others will with 10. Certain observers appear to excel in detecting delicate planetary markings, while others possess special aptitude for glimpsing minute objects such as faint satellites, or comites to double stars, and the explanation seems to be that partly by experience and partly by differences in the sensitiveness of vision, exceptional powers are sometimes acquired in each of these departments. The various test-objects which have been given by reliable authorities, though representing average attainments, are not applicable to the abnormal powers of vision possessed by certain observers. In fact, the capacity of a telescope cannot be correctly assigned and its powers circumscribed by arbitrary rules, because, as already stated, the character of the observer himself becomes a most important factor in this relation. Climatic influences have also considerable weight, though less so than the personal variations referred to, for one man will succeed, where another meets with utter failure. This is unquestionably due to differences in eyesight, method, and experience. But whatever the primary causes may be, everyone knows they induce widely discordant results, and occasion many of the contradictions which become the subjects of controversy. And, as a rule, amateurs should avoid controversy, because it very rarely clears up a contested point. There is argument and reiteration, but no mutual understanding or settlement of the question at issue. It wastes time, and often destroys that good feeling which should subsist amongst astronomers of every class and nationality. In cases where an important principle is involved, and discussion promises to throw light upon it, the circumstances are quite different. But paltry quibblings, fault-finding, or the constant expression of negative views, peculiar to sceptics, should be abandoned, as hindering rather than accelerating the progress of science. Let observers continually exercise care and discretion and satisfy themselves in every legitimate way as to the accuracy of their results, and they may fearlessly give them expression and overcome any objections made to their acceptance. They should accord one another an equal desire for the promotion of truth. Competition and rivalry in good spirit increase enthusiasm, but there is little occasion for the bitterness and spleen sometimes exhibited in scientific journals. There are some men whose reputations do not rest upon good or original work performed by themselves, but rather upon the alacrity with which they discover grievances and upon the care they will bestow in exposing trifling errors in the writings of their not-infallible contemporaries. Such critics would earn a more honourable title to regard were they to devote their time to some better method of serving the cause of science.
Cheapness and increasing number of Telescopes.—A marked feature of optical instruments is their increasing cheapness. Little more than half a century ago Tulley charged £315 for a 10-inch Newtonian reflector. At the present time Calver asks £50 for an instrument of the same aperture, and sometimes one may be picked up, second-hand, for half of that amount. Not only have telescopes become cheaper, but they have greatly improved in performance since silvered glass superseded the metallic speculum. Hence we find moderately-powerful instruments in the hands of a very large number of observers. Astronomical publications have proportionately increased, so that amateurs of to-day can boast of facilities, both of making and recording observations, which were scarcely dreamt of a century ago. It must be admitted, however, that the results hardly do justice to the means available. Such an enormous number of telescopes are variously employed that one cannot avoid a feeling of surprise at the comparative rarity of new discoveries, and, indeed, of published observations generally. It is certain that the majority of existing telescopes are either lying idle or applied in such a desultory fashion as to virtually negative the value of the results. Others, again, are indiscriminately employed upon every diversity of object without special aim or method, and with a mere desire to satisfy curiosity. Now it is to be greatly deplored that so much observing strength is either latent or misdirected. The circumstances obviously demand that an earnest effort should be made to utilize and attract it into suitable channels. To do this effectually, the value of collective effort should be forcibly explained, the interest and enthusiasm of observers must be aroused in a permanent manner, and they must be banded together according to their choice of subjects. An effort in this direction has been made by the Liverpool Astronomical Society, and the results have proved distinctly favourable; a considerable amount of useful work has been effected in several branches and it forms the subject of some valuable reports which have been annually published in the ‘Journal.’
Utility