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as Venus entered on the solar disc, the sweep of light round the dark disc of Venus would enable a very precise observation to be made. The Transit of Venus in 1874, in which the present writer assisted, overthrew this delusion. In 1877 Sir David Gill used Lord Crawford's heliometer at the Island of Ascension to measure the parallax of Mars in opposition, and found the sun's distance 93,08o,ooo miles. He considered that, while the superiority of the heliometer had been proved, the results would be still better with the points of light shown by minor planets rather than with the disc of Mars. In 1888–9, at the Cape, he observed the minor planets Iris, Victoria, and Sappho, and secured the co-operation of four other heliometers. His final result was 92,870, ooo miles, the parallax being 8”,802 (Cape Obs., Vol. VI.). So delicate were these measures that Gill detected a minute periodic error of theory of twenty-seven days, owing to a periodically erroneous position of the centre of gravity of the earth and moon to which the position of the observer was referred. This led him to correct the mass of the moon, and to fix its ratio to the earth's mass = o.o.1 2240. Another method of getting the distance from the sun is to measure the velocity of the earth's orbital motion, giving the circumference traversed in a year, and so the radius of the orbit. o This has been done by comparing observation and experiment. The aberration of light is an angle 2 o'' 48, giving the ratio of the earth's velocity to the velocity of light. The velocity of light is 186, ooo miles a second; whence the distance to the sun is 92,78o, ooo miles. There seems, however, to be some uncertainty about the true value of the aberration, any determination of which is subject to irregularities due to the “seasonal errors.” The velocity of light was experimentally found, in 1862, by Fizeau and Foucault, each using an independent method. These methods have been developed, and new values found, by Cornu, Michaelson, Newcomb, and the present writer. 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." But the remarkably erratic minor planet, Eros, discovered by Witte in 1898, approaches the earth within 15, ooo, ooo 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 opposi
tion of 1900. Their results are not yet completely reduced, but the best value deduced so far for the parallax' is 8”.807 to".oo28.”
II. 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,” 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-glass and a concave eye-lens, and this type is spoken of as the Galilean telescope. Its defects are that it * The parallax of the sun is the angle subtended by the earth's radius at the sun's distance. * A. R. Hinks, R.A.S.; Monthly Notices, June, 1909. * In the Encyclopædia Britannica, article “ Tele
scope,” and in Grant's Physical Astronomy, good reasons are given for awarding the honour to Lipperhey. 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 3oo 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," 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. * 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 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 glass, the other of flint glass — 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-glass 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-glass 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