The opening years of the seventeenth century found the world without a telescope, or, at least, such an instrument as was adapted for astronomical work. It is true that long years before, Arabian and some other eastern astronomers, for the purpose, possibly, of enabling them to concentrate their gaze upon celestial objects and follow their motions, had been accustomed to use a kind of tube consisting of a long cylinder without glasses of any kind and open at both ends. For magnifying purposes, this tube was of no value. Still, it must have been of some kind of service, or else the first telescopes, as constructed by the spectacle makers, who had stumbled upon the principle involved, were exceedingly sorry affairs, for, soon after their introduction, the illustrious Kepler, in his work on "Optics," recommended the employment of plain apertures, without lenses, because they were superior to the telescope on account of their freedom from refraction.
But as soon as the principle by which distant objects could, apparently, be brought nearer the eye became known and its value recognized by philosophers, telescopes ceased to be regarded as toys, and underwent material improvements in the hands of such men as Galilei, and, later, even of Kepler himself, Cassini, Huyghens, and others. Galilei's first telescope magnified but three times, and his best not much above thirty times. If I comprehend aright what has been written upon the subject, I am justified in saying that this little instrument in my hand, with an aperture of one inch and one-quarter, and a focus, with an astronomical eye-piece, of about ten inches, is a better magnifier than was Galilei's best. With it I can see the moons of Jupiter, some spots on the sun, the phases of Venus, the composition, in some places, of the Milky Way, the seas, the valleys, the mountains, and, when in bold relief upon the terminator, even some of the craters and cones of the moon. Indeed, I am of opinion I can see even more than he could, for I can readily make out a considerable portion of the Great Nebula in Orion, some double stars, and enough of the Saturnian system to discern the disk of the planet and see that there is something attached to its sides.
For nearly one hundred and fifty years all refracting telescopes labored under one serious difficulty. The images formed by them were more or less confused by rainbow tints, due to the bending, or refracting, by the object glass of the rays of light. To overcome this obstacle to clear vision, and also to secure magnification, the focal lengths of the instruments were greatly extended. Telescopes 38, 50, 78, 130, 160, 210, 400, and even 600 feet long were constructed. I can, however, find nothing on record indicating that the object glasses of these enormously attenuated instruments ever exceeded in diameter two and one-half inches. Yet, with unwieldy and ungainly telescopes, nearly always defining badly, wonders were accomplished by the painstaking and indomitable observers of the time.
In 1658, Huyghens, using a telescope twenty-three feet long and two and one-third inches in diameter, with a power of 100, solved the mystery of Saturn's rings, which had resisted all of Galilei's efforts as well as his own with a shorter instrument, though he had discovered Titan, Saturn's largest moon, and fixed correctly its period of revolution at sixteen days. Fifteen years later, Ball, with a telescope thirty-eight feet long, discovered the principal division in the rings. Ten years still later, Cassini, with an instrument twenty feet long and an object glass two and one-half inches in diameter, rediscovered the division, which was named after him, rather than after Ball, who had taken no pains to make widely known his discovery, which, in the meantime, had been forgotten. Though we have no record, there is no doubt that the lamented Horrocks and Crabtree, in England, in 1639, with glasses no better than these, watched with exultant emotions the first transit of Venus ever seen by human eyes.
In 1722, Bradley, with a telescope 223¼ feet long, succeeded in measuring the diameter of the same planet. Yet Grant assures us that, in spite of all their difficulties, such was the industry of the astronomers that when, at the commencement of this century, it became possible to construct larger refracting telescopes, there was nothing to be discovered that could have been discovered with the means at their disposal. So far as we now know, a good three-inch telescope, nay, a first-rate two inch one, will show far more than our great-grandfathers ever saw, or dreamed of seeing, with their refractors.
Toward the middle of the seventeenth century the reflecting telescope had been so much improved as nearly to crowd out its refracting rival, but, just as its success seemed to be assured, Dollond, working along lines partially followed up by Hall, found a combination of lenses by which the chromatic aberration of the refractor could be very perfectly corrected. While Dollond's invention was of immense value, it remained that flint object glasses larger than two and one-half inches in diameter could not, for some years, be manufactured, but about the opening of the nineteenth century, Guinand, a Swiss, discovered a process of making masses of optical flint glass sufficiently large as to admit of the construction from them of excellent lenses of sizes gradually increasing as time and experimenting went on. The making of three-inch objectives, achromatic and of short focus, wrought a revolution in telescopes and renewed the demand for refractors, though prices, as compared with those of the present day, were very great. But improvement was succeeded by improvement. Larger and still larger objectives were made, yet progress was not so rapid as not to justify Grant, in 1852, in declaring to be a "munificent gift" the presentation, about 1838, to Greenwhich Observatory, of a six and seven-tenths object glass alone, and so it was esteemed by Mr. Airy, the astronomer royal. Improvement is still the order of the day, and, as a result of keen competition, very excellent telescopes of small aperture can be purchased at reasonable prices. Great telescopes are enormously expensive, and will probably be so until they are superseded by some simple invention which shall be as superior to them as they are to the "mighty" instruments which, from time to time, caused such sensations in the days of Galilei, Cassini, Huyghens, Bradley, Dollond, and those who came after them.
But, notable as are the services rendered to science by giant telescopes, it remains that by far the greater bulk of useful work has been done by apertures of less than twelve inches in diameter. Indeed, it may be asserted that most of such work has been done by instruments of six inches or less in size. After referring with some detail to this, Denning tells us that "nearly all the comets, planetoids, double stars, etc., owe their detection to small instruments; that our knowledge of sun spots, lunar and planetary features is also very largely derived from similar sources; that there is no department which is not indebted to the services of small telescopes, and that of some thousands of drawings of celestial objects, made by observers employing instruments from three to seventy-two inches in diameter, a careful inspection shows that the smaller instruments have not been outdone in this interesting field of observation, owing to their excellent defining powers and the facility with which they are used." Aperture for aperture, the record is more glorious for the "common telescope" than for its great rivals. Let us for a moment recall something of what has been done with instruments which may be embraced under the designation "common" as such a statement may serve to remove impressions that small telescopes are but of little use in astronomical work.
In his unrivaled book, Webb declares that his observations were chiefly made with a telescope five and one-half feet long, carrying an object glass of a diameter of three and seven-tenths inches. The instrument was of "fair defining quality," and one has but to read his delightful pages in order to form an idea of the countless pleasures Webb derived from observation with it. Speaking of it, he says that smaller ones will, of course, do less, especially with faint objects, but are often very perfect and distinct, and that even diminutive glasses, if good, will, at least, show something never seen without them. He adds: "I have a little hand telescope twenty-two and one-quarter inches long, when fully drawn out, with a focus of about fourteen inches, and one and one-third inches aperture; this, with an astronomical eye-piece, will show the existence of sun spots, the mountains in the moon, Jupiter's satellites and Saturn's ring." In another place, speaking of the sun, he says that an object glass of only two inches will exhibit a curdled or marbled appearance over the whole solar disk, caused by the intermixture of spaces of different brightness. And I may add here that Dawes recommends a small aperture for sun work, including spectroscopic examinations, he himself, like Mr. Miller, our librarian, preferring to use for that purpose a four inch refractor.
As you know, the North Star is a most beautiful double. Its companion is of the ninth order of magnitude, that is, three magnitudes smaller than the smallest star visible to the naked eye on a dark night. There was a time when Polaris, as a double, was regarded as an excellent test for a good three inch telescope; that is any three inch instrument in which the companion could be seen was pronounced to be first-class. But so persistently have instruments of small aperture been improved that that star is no longer an absolute test for three inch objectives of fine quality, or any first-rate objective exceeding two inches for which Dawes proposed it as a standard of excellence, he having found that if the eye and telescope be good, the companion to Polaris may be seen with such an aperture armed with a power of eighty. As a matter of fact, Dawes, who was, like Burnham, blessed with most acute vision, saw the companion with an instrument no larger than this small one in my hand—one inch and three-tenths. Ward saw it with an inch and one-quarter objective, and Dawson with so small an aperture as one inch. T.T. Smith has seen it with a reflector stopped down to one inch and one-quarter, while in the instrument still known as the "great Dorpat reflector," it has been seen in broad daylight. This historic telescope has, I believe, a twelve inch object glass, but the difficulty of seeing in sunshine so minute a star is such that the fact may fairly be mentioned here.
Another interesting feature is this. Objects once discovered, though thought to be visible in large telescopes only, may often be seen in much smaller ones. The first Herschel said truly that less optical power will show an object than was required for its discovery. The rifts, or canals, in the Great Nebula in Andromeda is a case in point, but two better illustrations may be taken from the planets. Though Saturn was for many years subjected to most careful scrutiny by skilled astronomers using the most powerful telescopes in existence, the crape ring eluded discovery until November, 1850, when it was independently seen by Dawes, in England, and Bond, in the United States. Both were capital observers and employed excellent instruments of large aperture, and it was naturally presumed that only such instruments could show the novel Saturnian feature. Not so. Once brought to the attention of astronomers, Webb saw the new ring with his three and seven-tenths telescope and Ross with an aperture not exceeding three and three-eighths in diameter. Nay, I am permitted to say that a venerable member of this society made drawings of it with a three inch refractor. With a two inch objective, Grover not only saw the crape ring, but Saturn's belts, as well, and the shadow cast by the ball of the planet upon its system of rings. Titan, Saturn's largest moon, is merely a point of light as compared with the planet, as it appears in a telescope, yet it has been seen, so it is said, with a one inch glass. The shadow of this satellite, while crossing the face of Saturn, has been observed by Banks with a two and seven-eighths objective. By hiding the glare of the planet behind an occulting bar, some of Saturn's smallest moons were seen by Kitchener with a two and seven-tenths aperture and by Capron with a two and three-fourths one. Banks saw four of them with a three and seven-eighths telescope, Grover two of them with a three and three-quarter inch, and four inches of aperture will show five of them, so Webb says. Rhea, Dione and Tethys are more minute than Japetus, yet Cassini, with his inferior means, discerned them and traced their periods. Take the instance of Mars next. It was long believed that Mars had no satellites. But in 1877, during one of the highly favorable oppositions of that planet which occur but once in about sixteen years, the able Hall, using the great 26 inch refractor at Washington, discovered two tiny moons which had never been seen before. One of these, called Deimos, is only six miles in diameter, the other, named Phobos, is only seven, and both are exceedingly close to the primary and in rapid revolution. The diameter of these satellites is really less than the distance from High Park, on the west of Toronto, to Woodbine race course, on the east of the city. No wonder these minute objects—seldom, if ever, nearer to us than about forty millions of miles—are difficult to see at all. Newcomb and Holden tell us that they are invisible save at the sixteen year periods referred to, when it happens that the earth and Mars, in their respective orbits, approach each other more nearly than at any other time. But once discovered, the rule held good even in the case of the satellites of Mars. Pratt has seen Deimos, the outermost moon, with an eight and one-seventh inch telescope; Erek has seen it with a seven and one-third inch achromatic; Trouvellot, the innermost one, with a six and three-tenths glass, while Common believes that any one who can make out Enceladus, one of Saturn's smallest moons, can see those of Mars by hiding the planet at or near the elongations, and that even our own moonlight does not prevent the observations being made. It chances for the benefit of observers, in the northern hemisphere especially, that one of the sixteen year periods will culminate in 1893, when Mars will be most advantageously situated for close examination. No doubt every one will avail himself of the opportunity, and may we not reasonably hope that scores of amateur observers throughout the United States and Canada will experience the delight of seeing and studying the tiny moons of our ruddy neighbor?