History of Astronomy Part 7

Quite lately Halm, at the Cape of Good Hope, measured spectroscopically the velocity of the earth to and from a star by observations taken six months apart. Thence he obtained an accurate value of the sun's distance.[5]

But the remarkably erratic minor planet, Eros, discovered by Witte in 1898, approaches the earth within 15,000,000 miles at rare intervals, and, with the aid of photography, will certainly give us the best result. A large number of observatories combined to observe the opposition of 1900. Their results are not yet completely reduced, but the best value deduced so far for the parallax[6] is 8".807 0".0028.[7]

FOOTNOTES:

[1] In 1480 Martin Behaim, of Nuremberg, produced his _astrolabe_ for measuring the lat.i.tude, by observation of the sun, at sea. It consisted of a graduated metal circle, suspended by a ring which was pa.s.sed over the thumb, and hung vertically. A pointer was fixed to a pin at the centre. This arm, called the _alhidada_, worked round the graduated circle, and was pointed to the sun. The alt.i.tude of the sun was thus determined, and, by help of solar tables, the lat.i.tude could be found from observations made at apparent noon.

[2] See ill.u.s.tration on p. 76.

[3] See Dreyer's article on these instruments in _Copernicus_, Vol. I. They were stolen by the Germans after the relief of the Emba.s.sies, in 1900. The best description of these instruments is probably that contained in an interesting volume, which may be seen in the library of the R. A. S., ent.i.tled _Chinese Researches_, by Alexander Wyllie (Shanghai, 1897).

[4] Sir George Airy was very jealous of this honourable t.i.tle. He rightly held that there is only one Astronomer Royal at a time, as there is only one Mikado, one Dalai Lama. He said that His Majesty's Astronomer at the Cape of Good Hope, His Majesty's Astronomer for Scotland, and His Majesty's Astronomer for Ireland are not called Astronomers Royal.

[5] _Annals of the Cape Observatory_, vol. x., part 3.

[6] The parallax of the sun is the angle subtended by the earth's radius at the sun's distance.

[7] A. R. Hinks, R.A.S.; _Monthly Notices_, June, 1909.

11. HISTORY OF THE TELESCOPE

Accounts of wonderful optical experiments by Roger Bacon (who died in 1292), and in the sixteenth century by Digges, Baptista Porta, and Antonio de Dominis (Grant, _Hist. Ph. Ast_.), have led some to suppose that they invented the telescope. The writer considers that it is more likely that these notes refer to a kind of _camera obscura_, in which a lens throws an inverted image of a landscape on the wall.

The first telescopes were made in Holland, the originator being either Henry Lipperhey,[1] Zacharias Jansen, or James Metius, and the date 1608 or earlier.

In 1609 Galileo, being in Venice, heard of the invention, went home and worked out the theory, and made a similar telescope. These telescopes were all made with a convex object-gla.s.s and a concave eye-lens, and this type is spoken of as the Galilean telescope. Its defects are that it has no real focus where cross-wires can be placed, and that the field of view is very small. Kepler suggested the convex eye-lens in 1611, and Scheiner claimed to have used one in 1617. But it was Huyghens who really introduced them. In the seventeenth century telescopes were made of great length, going up to 300 feet. Huyghens also invented the compound eye-piece that bears his name, made of two convex lenses to diminish spherical aberration.

But the defects of colour remained, although their cause was unknown until Newton carried out his experiments on dispersion and the solar spectrum. To overcome the spherical aberration James Gregory,[2] of Aberdeen and Edinburgh, in 1663, in his _Optica Promota_, proposed a reflecting speculum of parabolic form. But it was Newton, about 1666, who first made a reflecting telescope; and he did it with the object of avoiding colour dispersion.

Some time elapsed before reflectors were much used. Pound and Bradley used one presented to the Royal Society by Hadley in 1723. Hawksbee, Bradley, and Molyneaux made some. But James Short, of Edinburgh, made many excellent Gregorian reflectors from 1732 till his death in 1768.

Newton's trouble with refractors, chromatic aberration, remained insurmountable until John Dollond (born 1706, died 1761), after many experiments, found out how to make an achromatic lens out of two lenses--one of crown gla.s.s, the other of flint gla.s.s--to destroy the colour, in a way originally suggested by Euler. He soon acquired a great reputation for his telescopes of moderate size; but there was a difficulty in making flint-gla.s.s lenses of large size. The first actual inventor and constructor of an achromatic telescope was Chester Moor Hall, who was not in trade, and did not patent it. Towards the close of the eighteenth century a Swiss named Guinand at last succeeded in producing larger flint-gla.s.s discs free from striae. Frauenhofer, of Munich, took him up in 1805, and soon produced, among others, Struve's Dorpat refractor of 9.9 inches diameter and 13.5 feet focal length, and another, of 12 inches diameter and 18 feet focal length, for Lamont, of Munich.

In the nineteenth century gigantic _reflectors_ have been made. La.s.sel's 2-foot reflector, made by himself, did much good work, and discovered four new satellites. But Lord Rosse's 6-foot reflector, 54 feet focal length, constructed in 1845, is still the largest ever made. The imperfections of our atmosphere are against the use of such large apertures, unless it be on high mountains.

During the last half century excellent specula have been made of silvered gla.s.s, and Dr. Common's 5-foot speculum (removed, since his death, to Harvard) has done excellent work. Then there are the 5-foot Yerkes reflector at Chicago, and the 4-foot by Grubb at Melbourne.

Pa.s.sing now from these large reflectors to refractors, further improvements have been made in the manufacture of gla.s.s by Chance, of Birmingham, Feil and Mantois, of Paris, and Schott, of Jena; while specialists in grinding lenses, like Alvan Clark, of the U.S.A., and others, have produced many large refractors.

Cooke, of York, made an object-gla.s.s, 25-inch diameter, for Newall, of Gateshead, which has done splendid work at Cambridge. We have the Was.h.i.+ngton 26-inch by Clark, the Vienna 27-inch by Grubb, the Nice 29-inch by Gautier, the Pulkowa 30-inch by Clark. Then there was the sensation of Clark's 36-inch for the Lick Observatory in California, and finally his _tour de force_, the Yerkes 40-inch refractor, for Chicago.

At Greenwich there is the 28-inch photographic refractor, and the Thompson equatoreal by Grubb, carrying both the 26-inch photographic refractor and the 30-inch reflector. At the Cape of Good Hope we find Mr. Frank McClean's 24-inch refractor, with an object-gla.s.s prism for spectroscopic work.

It would be out of place to describe here the practical adjuncts of a modern equatoreal--the adjustments for pointing it, the clock for driving it, the position-micrometer and various eye-pieces, the photographic and spectroscopic attachments, the revolving domes, observing seats, and rising floors and different forms of mounting, the siderostats and coelostats, and other convenient adjuncts, besides the registering chronograph and numerous facilities for aiding observation. On each of these a chapter might be written; but the most important part of the whole outfit is the man behind the telescope, and it is with him that a history is more especially concerned.

SPECTROSCOPE.

Since the invention of the telescope no discovery has given so great an impetus to astronomical physics as the spectroscope; and in giving us information about the systems of stars and their proper motions it rivals the telescope.

Frauenhofer, at the beginning of the nineteenth century, while applying Dollond's discovery to make large achromatic telescopes, studied the dispersion of light by a prism. Admitting the light of the sun through a narrow slit in a window-shutter, an inverted image of the slit can be thrown, by a lens of suitable focal length, on the wall opposite. If a wedge or prism of gla.s.s be interposed, the image is deflected to one side; but, as Newton had shown, the images formed by the different colours of which white light is composed are deflected to different extents--the violet most, the red least. The number of colours forming images is so numerous as to form a continuous spectrum on the wall with all the colours--red, orange, yellow, green, blue, indigo, and violet. But Frauenhofer found with a narrow slit, well focussed by the lens, that some colours were missing in the white light of the sun, and these were shown by dark lines across the spectrum. These are the Frauenhofer lines, some of which he named by the letters of the alphabet. The D line is a very marked one in the yellow. These dark lines in the solar spectrum had already been observed by Wollaston. [3]

On examining artificial lights it was found that incandescent solids and liquids (including the carbon glowing in a white gas flame) give continuous spectra; gases, except under enormous pressure, give bright lines. If sodium or common salt be thrown on the colourless flame of a spirit lamp, it gives it a yellow colour, and its spectrum is a bright yellow line agreeing in position with line D of the solar spectrum.

In 1832 Sir David Brewster found some of the solar black lines increased in strength towards sunset, and attributed them to absorption in the earth's atmosphere. He suggested that the others were due to absorption in the sun's atmosphere. Thereupon Professor J. D. Forbes pointed out that during a nearly total eclipse the lines ought to be strengthened in the same way; as that part of the sun's light, coming from its edge, pa.s.ses through a great distance in the sun's atmosphere. He tried this with the annular eclipse of 1836, with a negative result which has never been accounted for, and which seemed to condemn Brewster's view.

In 1859 Kirchoff, on repeating Frauenhofer's experiment, found that, if a spirit lamp with salt in the flame were placed in the path of the light, the black D line is intensified. He also found that, if he used a limelight instead of the sunlight and pa.s.sed it through the flame with salt, the spectrum showed the D line black; or the vapour of sodium absorbs the same light that it radiates. This proved to him the existence of sodium in the sun's atmosphere.[4] Iron, calcium, and other elements were soon detected in the same way.

Extensive laboratory researches (still incomplete) have been carried out to catalogue (according to their wave-length on the undulatory theory of light) all the lines of each chemical element, under all conditions of temperature and pressure. At the same time, all the lines have been catalogued in the light of the sun and the brighter of the stars.

Another method of obtaining spectra had long been known, by transmission through, or reflection from, a grating of equidistant lines ruled upon gla.s.s or metal. H. A. Rowland developed the art of constructing these gratings, which requires great technical skill, and for this astronomers owe him a debt of grat.i.tude.

In 1842 Doppler[5] proved that the colour of a luminous body, like the pitch or note of a sounding body, must be changed by velocity of approach or recession. Everyone has noticed on a railway that, on meeting a locomotive whistling, the note is lowered after the engine has pa.s.sed. The pitch of a sound or the colour of a light depends on the number of waves striking the ear or eye in a second. This number is increased by approach and lowered by recession.

Thus, by comparing the spectrum of a star alongside a spectrum of hydrogen, we may see all the lines, and be sure that there is hydrogen in the star; yet the lines in the star-spectrum may be all slightly displaced to one side of the lines of the comparison spectrum. If towards the violet end, it means mutual approach of the star and earth; if to the red end, it means recession. The displacement of lines does not tell us whether the motion is in the star, the earth, or both. The displacement of the lines being measured, we can calculate the rate of approach or recession in miles per second.

In 1868 Huggins[6] succeeded in thus measuring the velocities of stars in the direction of the line of sight.

In 1873 Vogel[7] compared the spectra of the sun's East (approaching) limb and West (receding) limb, and the displacement of lines endorsed the theory. This last observation was suggested by Zollner.

FOOTNOTES:

[1] In the _Encyclopaedia Britannica_, article "Telescope," and in Grant's _Physical Astronomy_, good reasons are given for awarding the honour to Lipperhey.

[2] Will the indulgent reader excuse an anecdote which may encourage some workers who may have found their mathematics defective through want of use? James Gregory's nephew David had a heap of MS. notes by Newton. These descended to a Miss Gregory, of Edinburgh, who handed them to the present writer, when an undergraduate at Cambridge, to examine. After perusal, he lent them to his kindest of friends, J. C. Adams (the discoverer of Neptune), for his opinion. Adams's final verdict was: "I fear they are of no value. It is pretty evident that, when he wrote these notes, _Newton's mathematics were a little rusty_."

[3] _R. S. Phil. Trans_.

[4] The experiment had been made before by one who did not understand its meaning;. But Sir George G. Stokes had already given verbally the true explanation of Frauenhofer lines.

[5] _Abh. d. Kon. Bohm. d. Wiss_., Bd. ii., 1841-42, p. 467. See also Fizeau in the _Ann. de Chem. et de Phys_., 1870, p. 211.

[6] _R. S. Phil. Trans_., 1868.

[7] _Ast. Nach_., No. 1, 864.

BOOK IV. THE PHYSICAL PERIOD

We have seen how the theory of the solar system was slowly developed by the constant efforts of the human mind to find out what are the rules of cause and effect by which our conception of the present universe and its development seems to be bound. In the primitive ages a mere record of events in the heavens and on the earth gave the only hope of detecting those uniform sequences from which to derive rules or laws of cause and effect upon which to rely. Then came the geometrical age, in which rules were sought by which to predict the movements of heavenly bodies. Later, when the relation of the sun to the courses of the planets was established, the sun came to be looked upon as a cause; and finally, early in the seventeenth century, for the first time in history, it began to be recognised that the laws of dynamics, exactly as they had been established for our own terrestrial world, hold good, with the same rigid invariability, at least as far as the limits of the solar system.