TELESCOPE, an optical instrument to assist the naked eye in examining distant objects. It does this in two ways—by magnifying the apparent angular dimen sions of the object, and by collecting more light than the pupil of the eye could alone do to form the image on the retina. One point should be noted that is often misunderstood. If any object is large enough to be seen distinctly as a surface by the naked eye, it cannot be made to appear any brighter in a telescope; in fact, it will always be fainter on account of the loss of light by reflection, and ab sorption in the object glass and eye piece. If, however, the object is faint and rather small to the naked eye, like a dis tant clock face in the twilight, it can be read more easily in a telescope. If the person should approach the clock face till it subtended the same angular dimen sions as its image in the telescope, he could read it more distinctly, and it would appear more brightly illuminated than in the telescope at the fist distance, provided he shielded his eyes from sur rounding stray light, which is another way in which the telescope sometimes helps in seeing faint objects. If, how ever, the sources of light examined are from points, like the apparent images of a star, which remain practically points under all magnifying powers, then these images will appear more and more bright the larger the aperture of the telescope, provided the whole of the emergent pencil of rays from the object glass enters the pupil of the eye. With large telescopes and low magnifying powers the emergent pencils are generally too large for this.
Essential Parts.—The essential parts of a telescope are: (1) Either a concave mirror or a system of lenses, called the objective, for bringing to a point in the focal plane the cone of rays pro ceeds from each point of the luminous object and falls upon the objective (for objects at a very great—practically in finite—distance these cones of rays that fall upon the objective may be regarded as cylinders). (2) An eye piece consist ing of one or more lenses for examining and magnifying the image formed in the focal plane of the objective. (3) A tube or framework of some kind to hold the objective and the eye piece in their proper relative positions. These are all
the essential parts of the simple telescope itself, regarded as an optical instrument only. But for the purposes of convenient use many other accessories of mounting and conveniences of handling are re quired. Moreover, a telescope fitted only as an optical instrument, for looking at objects simply, is of very little scientific value in these modern days of exact mea surement and quantitative determination of everything examined, so that in many forms of astronomical, geodetic, and la boratory instruments the optical tele scope or microscope is only one of the important parts of the whole, the acces sory apparatus attached to it, or to which it is attached, constituting in some in struments the more important feature of the whole. Moreover, for purposes of greater convenience in handling and pointing or of comfort to the observer, many considerable modifications of the optical part itself are introduced, as in the "broken back" transit, where the cone of rays from the objective of the eye piece is reflected at right angles in its course and brought out at one end of the axis, where the observer works in one position without having to follow the rev olutions of the eye piece, as in the ordi nary form. Also in the equatorial-coude, or elbow-equatorial, first equatorial tele scope introduced at the Paris Observa tory by Loewy, the observer sits in one position at the upper end of the equa torial axis, which is itself a part of the telescope tube, the rest of it branching off at right angles from the lower end with the objective at the outer end of this branch, with a large mirror in front of this objective turning on an axis which enables the instrument to be pointed to any polar distance, and the motion of the whole round the polar axis tube reaching any hour angle from the merid ian. Another mirror at the lower end of this axis reflects the rays up to the focal plane at the upper end, where the observer at the eye piece has the whole instrument under his control, and works as he would at a microscope, having, moreover, this end inclosed in a warmed room built round the eye piece end if he so desires.