TELESCOPE. 107 TELESCOPE. which reflected the rays to the eyepiece, as is shown in the following diagram: DIAQBAM SHOWING FORMATION OF AN IMAGE BY A CONCAVE MIRKOR. C is a concave mirror on which rays from a point of a distant object, A, fall. Following the law of reflection, these rays will be reflected and will be nnited at B. tlie focus for rays from the given point. The object A is made up of a num- ber of such points: consequently at B there will be a number of points where the rays from A are collect^'d and an image of the original object will be formed. If the rays come from an object infinitely distant, or in other words are parallel, they will converge at the principal focus, which is one-half of the radius of curvature. This will be the case in a mirror of large radius and for rays that are incident, not far from the axis; but if the mirror is large, then the rays near its cir- cumference will not be brought to a focus at the point B. but nearer to the mirror, and conse- quently a blurreil image will be produced. This is known as spherical aberration and is discussed under that head. Theoretically this could be remedied by the construction of a mirror of para- bolic section, as the configuration of such a mirror is such that all parallel rays are brought to a focus in the same point. In practice, how- ever, this is extremely diflicult of attainment, as to secure a properly shaped mirror the most minute amounts of material must be removed from a spherical surface in order to make it parabolic. When an image is formed in this way it can be viewed either by being received on a screen or it can be observed with an eyepiece or a simple microscope. This is done in several w-ays in the reflecting telescope, and the chief points of difference in the various instruments are mainly in this respect. A concave mirror, according to tradition, was used by Ptolemy Eu- ergetes on a lighthouse at Alexandria to discover distant ships. The Romans were acquainted with the power of collecting or concentrating rays of light possessed by such a mirror, but there does not seem to be any well-authenticated record of the application of the idea to the pur- poses of a telescope. Father Zucchi, an Italian Jesuit, was the first to use an eye lens to view the imago produced by a concave mirror (1616-16.52), but to Gregory is due the first description of a telescope with a reflecting mirror, and the instrument has since been known by his name. Gregory, along with others, realized the shortcomings of a telescope with lenses and believed that the manufacture ■of concave mirrors would be attended with far less difficulty. An actual working instrument Ijased on this principle was devised and con- structed by Isaac Newton. With a telescope formed by a mirror of 6^4 inches focal length, which magnified 3S times, he was enabled to make important observations. In these tele- scopes the great difficulty was viewing the image, as the eyepiece and the head of the observer would cut off a large portion of the incident rays. In the Gregorian telescope this was obviated by the interposition of a second concave mirror, Vol. XIX.— 8. GREGOBIAN TELKSCOI'E. Newton used a plane mirror placed at an angle of 45° to the axis, which reflected the rays into an eyepiece arranged as in the following dia- gram : NEWTONIAN TELESCOPE. Draper used a total reflection prism instead of the plane mirror with considerable success, be- ing one of the few astronomers in the United States to construct a reflecting mirror. Casse- grain employed a convex mirror instead of a con- cave one. Herschel obtained satisfactory results by tilt- ing his mirror and placing the eyepiece below the axis of the instrument, so that it was not in the HERSCHBL'S TELESCOPE. way of the incident rays. Herschel's mirrora were as large as four feet in diameter, with a tulie forty feet in length. The reflecting telescope was extensively used because there was no chromatic aberration caused by retraction. Spherical aberration was, however, present and was a serious drawback. According to geometrical calculations, as has been said, there would be no spherical aberration if a parabolic mirror was used, as all the rays from .a distant oliject would come to a focus at one point. The grinding of a parabolic mirror, however, was attended by many difliculties and was practically impossible except to a few opticians. Perhaps the most celebrated reflecting tele- scope was that of Lord Rosse, whose mirror w-as six feet across. This was completed in 1S42 and erected at Parsontown in Ireland. MiiIe fa- mous for its size, this telescope has never been used in making discoveries of prime importance, and has not been used during recent years, being so mounted as not to be available for photo- graphic work. Second to this, as will be seen from the table at the end of this article, is the reflector of Dr. A. A. Common at Ealing, Eng- land. Using the largest glass disk that could be obtained, five feet in" diameter. Dr. Common jiro- dnced a perfect mirror, which is mounted equato- rially. This telescope is of the Newtonian type and has been particularly useful in celestial photography. The mirrors for reflecting tele- scopes were usually made of speculum metal.
