The formation of an image by a concave mir ror has been made use of in the reflecting tele scope, of which numerous varieties have been devised and with which many of the most important astronomical discoveries have been made. The principle of this instrument will be apparent from the following diagram and ex planation: which reflected the rays to the eyepiece, as is shown in the following diagram: 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 he reflected and will be united at 13, the 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 collected 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 blurred 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 difficult 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 ways 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 image produced by a concave mirror (1616-1552), 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. based on this principle was devised and con structed by Isaac Newton. With a telescope formed by a mirror of inches focal length, which magnified 38 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.
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: 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 way of the incident rays. Herschel's mirrors were as large as four feet in diameter, with a tube forty feet in length.
The reflecting telescope was extensively used because there was no chromatic aberration caused by refraction. 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 object would come to a focus at one point. The grinding of a parabolic mirror, however, was attended by many difficulties and was practically impossible except to a few opticians.
Perhaps the most celebrated reflecting tele scope was that of Lord Bosse, whose mirror was six feet across. This was completed in 1342 and erected at Parsontown in Ireland. While 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. CO1111110/1 pro duced 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, which is composed of a mixture of copper and tin, until Liebig discovered the method of de positing a film of silver on a glass surface.
The use of silvered glass for mirrors was sug gested by Steinheil and later by Foucault, and has met with general adoption. as it not only fa cilitates the construction of the mirror, but makes possible its resilvering at any time with out the destruction of its configuration. The reflecting telescope is available for photographic or spectroscopic work, as well as for visual ob servation. but the chief defects are the difficulty of grinding the metal or glass to true parabolic shape and the deformation of the mirror in its mounting owing to its great mass. In recent years the most important use made of a reflect ing telescope has been to photograph the Nebuhe with the Crossley reflector of the Lick Obser vatory (q.v.), which is the largest telescope of this description in the United States. It was made by Dr. A. A. Common and was presented to Lick Observatory by Edward Crossley. Pro fessor James T. Keeler, the director, remounted this instrument and in 1899 made a remarkable series of photographs.