6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke & Sons.
We have now to consider the reflecting form of telescope, which, especially in this country, has deservedly gained much favour, and has come to be regarded as in some sense the amateur's particular tool.
FIG. 12.—8-INCH REFRACTOR ON EQUATORIAL MOUNTING.
As a matter of policy, one can scarcely advise the beginner to make his first essay with a reflector. Its adjustments, though simple enough, are apt to be troublesome at the time when everything has to be learned by experience; and its silver films, though much more durable than is commonly supposed, are easily destroyed by careless or unskilful handling, and require more careful nursing than the objective of a refractor. But, having once paid his first fees to experience, the observer, if he feel so inclined, may venture upon a reflector, which has probably more than sufficient advantages to make up for its weaker points. First and foremost of these advantages stands the not inconsiderable one of cheapness. A 10½-inch reflector may be purchased new for rather less than the sum which will buy a 4-inch refractor. True, the reflector has not the same command of light inch for inch as the refractor, but a reflector of 10½ inches should at least be the match of an 8-inch refractor in this respect, and will be immeasurably more powerful than the 4-inch refractor, which comes nearest to it in price. Second stands the ease and comfort so conspicuous in observing with a Newtonian. Instead of having almost to break his neck craning under the eye-piece of a telescope pointed to near the zenith, the observer with a Newtonian looks always straight in front of him, as the eye-piece of a reflector mounted as an altazimuth is always horizontal, and when the instrument is mounted equatorially, the tube, or its eye-end, is made to rotate so that the line of vision may be kept horizontal. Third is the absence of colour. Colour is not conspicuous in a small refractor, unless the objective be of very bad quality; but as the aperture increases it is apt to become somewhat painfully apparent. The reflector, on the other hand, is truly achromatic, and may be relied upon to show the natural tints of all objects with which it deals. This point is of considerable importance in connection with planetary observation. The colouring of Jupiter, for instance, will be seen in a reflector as a refractor can never show it.
Against these advantages there have to be set certain disadvantages. First, the question of adjustments. A small refractor requires practically none; but a reflector, whatever its size, must be occasionally attended to, or else its mirrors will get out of square and bad performance will be the result. It is easy, however, to make too much of this difficulty. The adjustments of the writer's 8½-inch With reflector have remained for months at a time as perfect as when they had been newly attended to. Second, the renewal of the silver films. This may cause some trouble in the neighbourhood of towns where the atmosphere is such as to tarnish silver quickly; and even in the country a film must be renewed at intervals. But these may be long enough. The film on the mirror above referred to has stood without serious deterioration for five years at a time. Third, the reflector, with its open-mouthed tube, is undoubtedly more subject to disturbance from air currents and changes of temperature, and its mirrors take longer to settle down into good definition after the instrument has been moved from one point of the sky to another. This difficulty cannot be got over, and must be put up with; but it is not very conspicuous with the smaller sizes of telescopes, such as are likely to be in the hands of an amateur at the beginning of his work. There are probably but few nights when an 8½-inch reflector will not give quite a good account of itself in this respect by comparison with a refractor of anything like equal power. On the whole, the state of the question is this: If the observer wishes to have as much power as possible in proportion to his expenditure, and is not afraid to take the risk of a small amount of trouble with the adjustments and films, the reflector is probably the instrument best suited to him. If, on the other hand, he is so situated that his telescope has to be much moved, or, which is almost as bad, has to stand unused for any considerable intervals of time, he will be well advised to prefer a refractor. One further advantage of the reflecting form is that, aperture for aperture, it is very much shorter. The average refractor will probably run to a length of from twelve to fifteen times the diameter of its objective. Reflectors are rarely of a greater length than nine times the diameter of the large mirror, and are frequently shorter still. Consequently, size for size, they can be worked in less space, which is often a consideration of importance.