TELESCOPE other lenses made by Borelli of 40 and 70 ft., and by Hartsoeker of not less than 250 ft. focus. These object glasses were used with- out any tube, the lens being placed upon a mast, or, as Cassini recommended, at the angle of a tower, and controlled, not without con- siderable difficulty, by cords leading to the observer at the eye lens. The source of the inconveniences attending the use of shorter lenses was generally supposed to lie wholly where it did really lie in part, in the imperfect collection of the rays of light, which were at that time believed to be homogeneous, into a simple focus. It was distinctly understood that the rays which passed through a lens near its centre would not be refracted to precisely the same point with those which pass through it near its circumference ; that is, there would be what is technically called spherical aberra- tion. This is a true cause, but by no means the whole cause of the indistinctness of images in the telescope. Accordingly, with that be- lief, it was thought the evil might be remedied by grinding lenses with other surfaces than spherical, and machines were devised by Des- cartes, by Hevelius of Dantzic, by Du Son of London (who ground deep parabolic concave lenses, with which he asserted that telescopes might be used " with full aperture," and yet show no colors), by Sir Christopher Wren, and others. But the main reliance of the as- tronomer until near the close of the century was in the aerial telescope, with which, un- wieldy as it was, many brilliant discoveries were made. An improvement, of more im- portance than that of the figuring of lenses, consisted in the modification of the eye piece. By the introduction of more than one convex lens, Rheita had reinverted the image ; but this was all the gain that either he or Kepler, who also proposed the same thing, seems to have expected. In fact, there was an increase of aberrations which caused distaste for the plan, and it was not until about 1659, when Huygens invented the combination which still bears his name, that much advantage was gained by multiplying lenses. This eye piece is composed of two convex lenses whose focal lengths are as 3 to 1, which are sepa- rated from each other by an interval equal to half the sum of these focal lengths, the place of the telescopic image being between the lenses. This arrangement was found to have a remarkable advantage in point of distinctness over the single eye glass, by rea- son of the apportionment of spherical aberra- tions between the lenses, and the consequent less amount of injurious effect in the result, while no addition whatever was made to the color of the images formed by the object glass. To this day the " Huygenian eye piece" re- mains one of the best combinations for or- dinary viewing purposes. Another eye piece, less successful, was constructed by Carnpani with three lenses so arranged as to show objects " vithout any iris or rainbow colors." The refracting telescope remained full three quar- ters of a century without further material im- provement. Morin, professor of mathematics in the college de France, first in 1634 attached a telescope to the moving index of a graduated arc, in order, as he says, " to measure the fixed stars quickly arid accurately." He was also the first to gain sight of stars in the daytime. But it was only after the introduction of fixed threads into the field of the telescope that it became a really useful auxiliary to instruments of measurement. At the present day it seems at first strange that astronomers should have preferred the simple "sights" or "pinnules," with which they had always been accustomed to observe, to the far more accurate perception furnished us by the telescope ; and yet they, without any means of designating the centre of the field of view, and with only the feeble optical power at their command, were right in their preference. Even as late as 1673, Heve- lius argued earnestly in favor of the pinnules for observing, from a want of confidence in the new method. As early as 1641 Gascoigne, an accomplished young English astronomer, had applied fixed threads to the telescope, and had also invented the wire or filar micrometer. He perished at the battle of Marston Moor, and his invention, of which no account had been published, remained forgotten until near- ly 30 years after, when an opportunity for reclamation occurred upon the reinvention of the micrometer by Auzout. About the same period Roemer gave to the telescope one of its most important applications, by attaching to it an axis at right angles to its length, and pla- cing it so as to revolve in the plane of the meridian ; and shortly afterward Pi card in Paris and Flamsteed at Greenwich, following up this idea, commenced a new era in obser- vation. (See TEANSIT CIRCLE.) Mersenne, in his correspondence with Descartes, had be- fore 1639 suggested the practicability of using a concave mirror instead of the principal lens in the telescope. In 1663 James Gregory of Edinburgh published, in his Optica Promota, the plan of a reflecting telescope, consisting of a concave mirror, perforated in the centre, by which the rays were to be converged to a focus before it, and after crossing would be received upon a second small concave mirror, be reflected back by the latter, and, crossing again near the opening in the first reflector, would be there received by a lens and trans- mitted to the eye. The rays having crossed twice, objects would appear in their natural position. An unsatisfactory attempt was made to construct such a telescope. Newton now took up the study. He soon found the true cause of the prismatic colors, and concluded "that the perfection of telescopes was hith- erto limited, not so much for want of glasses truly figured according to the prescriptions of optic authors, ... as because that light itself is a heterogeneous mixture of differently re- frangible rays. So that, were a glass so exactly
