Louis Bell

“ If economy must be enforced it is much wiser to try to pick up a used instrument of first class manufacture than to chance a new one at a low price. Now and then a maker of very ordinary repute may turn out a good instrument, but the fact is one to be proved—not assumed. Age and use do not seriously deteriorate a telescope if it has been given proper care. Some of Fraunhofer’s are still doing good service after a century, and occasionally an instrument from one of the great makers comes into the market at a real bargain. ”

“ It is a curious and dramatic fact that to a single man was due not only the origin of the art but the optical glass industry of the world. If the capacity for taking infinite pains be genius, then the term rightfully belongs to Pierre Louis Guinand. He was a Swiss artisan living in the Canton of Neuchatel near Chaux-de-Fonds, maker of bells for repeaters, and becoming interested in constructing telescopes imported some flint glass from England and found it bad. ”

“ The path of the rays is plainly shown and the manner in which the prisms fold up the total focal length of the objective is quite obvious. The added stereoscopic effect obtained by the arrangement of the two sides of the instrument is practically a very material gain. It gives admirable modelling of the visible field, a perception of distance which is at least very noticeable and a certain power of penetration, as through a mass of underbrush, which results from the objectives to a certain extent seeing around small objects so that one or the other of them gives an image of something beyond. ”

“ Bad striæ are of course seen easily, as they would be in a window pane, but such gross imperfections are often in reality less damaging than the apparently slighter ones which must be searched for. The simplest test is to focus a good telescope on an artificial star, remove the eyepiece and bring the eye into its place.
When the eye is in focus the whole aperture of the objective is uniformly filled with light, and if the disc to be tested be placed in front of it, any inequality in refraction will announce itself by an inequality of illumination. ”

“ For casual observations and occasional use—all that most busy buyers of telescopes can expect—the refractor has a decided advantage in convenience. If one has leisure for frequent observations, and particularly if he can give his telescope a permanent mount, and is going in for serious work, he will do well not to dismiss the idea of a reflector without due deliberation.
In any case it is good policy to procure an instrument from one of the best makers. And if you do not buy directly of the actual maker it is best to deal with his accredited agents. ”

“ The telescope is the lifter of the cosmic veil, and even for merely disclosing the spectacular is a source of far-reaching enlightenment. But for the serious student it offers opportunities for the genuine advancement of human knowledge that are hard to underestimate. It is true that the great modern observatories can gather information on a scale that staggers the private investigator. But in this matter fortune favors the pertinacious, and the observer who settles to a line of deliberate investigation and patiently follows it is likely to find his reward. ”

“ Really very little can be done to the color correction without going to the new special glasses, the use of which involves other difficulties, and leaves the matter of adjustment for power quite in the air, to be brought down by special eye pieces. Now and then a melting of glass has a run of dispersion somewhat more favorable than usual, but there is small chance of getting large discs of special characteristics, and the maker has to take his chance, minute differences in chromatic quality being far less important than uniformity and good annealing. ”

“ The monocentric eyepiece has a high reputation for keen definition and is admirably achromatic and orthoscopic. The sharp field is about 32°, rather the largest given by any of the cemented combinations. All these optically single lenses are quite free of ghosts, reduce scattered light to a minimum, and leave little to be desired in precise definition. The weak point of the whole tribe is the small field, which, despite Herschel’s opinion, is a real disadvantage for certain kinds of work and wastes the observer’s time unless his facilities for close setting are more than usually good. ”

“ The eye being quite uncorrected for color the image produced even by a reflector is likely to show a colored fringe if at all bright, the more conspicuous as the relative aperture of the pupil increases. For low power eyepieces the emerging ray may quite fill a wide pupil and then the chromatic error is troublesome. Hence it has been the custom of skilled opticians, from the time of Fraunhofer, who probably started the practice, to overdo the correction of the objective just a little to balance the fault of the eye. ”

“ Hence there are times when decreasing the aperture of an objective by a stop improves the seeing considerably by increasing the relative length of the air waves. Such is in fact found to be the case in practical observing, especially when the seeing with a large aperture is decidedly poor. In other words one may often gain more by increased steadiness than he loses by lessened “resolving power,” the result depending somewhat on the class of observation which chances to be under way. ”

“ There is no reason to believe that the Dutch discovery, important as it was, had gone beyond the empirical observation that a common convex spectacle lens and a concave one of relatively large curvature could be placed in a tube, convex ahead, at such a distance apart as to give a clear enlarged image of distant objects. ”

“ Where nights, otherwise of the best seeing quality, are generally windless or with breezes so slight that the tube does not quiver a telescope in the open, however protected between times, works perfectly well.
In other regions the clearest nights are apt to be those of a steady gentle wind producing great uniformity of conditions at the expense of occasional vibration of the instrument and of discomfort to the observer. Hence one finds all sorts of practice, varied too, by the inevitable question of expense. ”

“ If the actual movements in the atmosphere could be made visible they would present a strange scene of turbulence—rushing currents taking devious courses up and around obstacles, slowly moving whirlpools, upward slants such as gulls hug on the quarter of a liner, great downward rushes dreaded by the aviator, and over it all incessant ripples in every direction. ”

“ Sometimes an objective will be found in which one edge of the focussed star image is notably red and the opposite one tinted with greenish or bluish, showing unsymmetrical coloring, still more obvious when the image is put a little out of focus. This means that the optical centers of crown and flint are out of line from careless edging of the lenses or very bad fitting. The case is bad enough to justify trying the only remedy available outside the optician’s workshop—rotating one lens upon the other and thus trying the pair in different relative azimuths. ”

“ The fact is that the ordinary stock telescope tripod is rather too high for sitting, and too low for standing, comfortably. A somewhat stubby tripod is advantageous both in point of steadiness and in accessibility of the eyepiece when one is observing within 30° of the zenith, where the seeing is at its best; and a sitting position gives a much greater range of convenient upward vision than a standing one.
When an equatorial mount is in use one faces the question of adjustment in its broadest aspect. ”

“ The loss of light in the prism glass is really a serious matter, between reflection at the surfaces and absorption in the thick masses of glass necessary in the prisms. If of any size the transmitted light is not much over one-half of that received, very seldom above 60%. If the instrument is properly designed the apparent field is in the neighborhood of 45°, substantially flat and fairly evenly illuminated. Warning should here be given however that many binoculars are on the market in which the field is far from flat and equally far from being uniform. ”

“ With a well mounted telescope of either sort a star at fair altitude on a fine night gives even better testing conditions than an artificial star, (Polaris is good in northern latitudes) but one may have a long wait.
If the reflector is of good figure and well adjusted, the star image, in focus or out, has quite the same appearance as in a refractor except that with a bright star in focus one sees a thin sharp cross of light centered on the image, rather faint but perfectly distinct. ”

“ It gives no ghosts, and the faint reflection at the eye end is widely spread so that if the exterior of the cylinder is well blackened, as it should be, it gives exceptional freedom from stray light. Still another variety of the Huygenian ocular sometimes useful is analogous to the compensating eyepiece used in microscopy. If, as commonly is the case, a telescope objective is over-corrected for color to correct for the chromatism of the eye in low powers, the high powers show strong over correction, the blue focus being longer than the red, and the blue image therefore the larger. ”

“ Often seeing conditions may be admirable save for this lack of transparency in the atmosphere, so that study of the moon, of planetary markings and even of double stars, not too faint, may go on quite unimpeded. The actual loss of light may reach however a magnitude or more, while the sky is quite cloudless and without a trace of fog or noticeable haziness by day. ”