Page:The New International Encyclopædia 1st ed. v. 19.djvu/131

* TELEPHUS. 105 TELESCOPE. the sea. The chest floated across the ^gean to the mouth of the Caeus, where Teuthras married Auge and brought up Telephus, who succeeded him on the throne. This early version was modified by the tragedians. Auge became priestess of Athena, and the child was born, or at any rate exposed, on Mount Parthenion, where it was suckled by a doe, and (in one version) found by Hercules. Auge meanwhile was cast into the sea and brought to Mysia, where she was adopted by Teuthras. Later, Telephus came to Mysia on account of an oracle, helped Teuthras against powerful enemies, and was rewarded by the hand of Auge. On the marriage night mother and son recognized each other. When the Greeks were on their way to Tro}', they landed by mistake in the territory of Telephus and harried the coun- try. Telephus defeated the invaders, but was himself wounded by Achilles. As an oracle in- formed him that only he who had wounded him could cure, he went to Greece, and as the Greeks needed his guidance to reach Troy, they yielded and Achilles healed the wound with rust from his spear. Telephus then guided the Greeks to Troy, set to work to produce such an instrument. His- telescope, similar in form to the modern opera glass (q.v.) and composed of a convex object glass and a concave eyepiece, as first constructed had a magnifying power of 3 times, but this was sub- sequently increased to 30 times, and Galileo was able to discover the satellites of Jupiter, the mountains of the moon, and other celestial objects. To Kepler we owe the discovery of the principle of the astronomical telescope with two convex lenses, and the description of such an instrument is contained in his Catoptrics (1611). This idea was actually employed in a telescope constructed by Father Scheiner {Rosa Vrsina, 1G30), and such telescopes were used in increasing numbers until the middle of the sev- enteenth century, when they were practically universal, and improved construction made pos- sible Huygens's discovery of Titan, the brightest satellite of Saturn. In order to appreciate the development of the telescope it may be worth while to consider some of the fundamental principles on which its action is based. A convex lens will give an image of a DIAGRAM OF ASTRONOMICAL TELESCOPE. but refused to take part in the war, since his wife, Astyoche, was a daughter (or sister) of Priam. The whole story of Telephus was repre- sented on the smaller frieze of the Great Altar of Pergamon, and his battle with Achilles in the west pediment of the Temple of Athena Alea at Tegea. Consult: Pilling, Quomodo Telephi fabu- lam et scriptores et artifices vetcres tractaverint (Halle, 1886) ; Jahn, Telephus und TroUos (Kiel, 1841) ; id., Telephus und TroUos und sein Ende (ib., 1859). TELESCOPE (from Gk. tijXwkottoi, tele- skopos, far-seeing, from rijXe, tile, afar + OKoireTv, skopein, to look). Essentially a lens or mirror to form an image of a distant object and a miscroscope to enable the observer to examine this image in detail. The invention of the tele- scope was doubtless accomplished in Holland, but there is no little confusion and controversy to be encountered in attempting to determine the origi- nal inventor. Tradition has it that about the beginning of the seventeenth century one Jansen, a spectacle-maker of Middleburg, Holland, con- structed a telescope about 16 inches in length which he exhibited to Prince Maurice and the Archduke Albert, who, appreciating the impor- tance of the discovery, paid him a sum of money to keep it concealed. Another spectacle-maker, Lippershey, made application (1608) to the States General for a patent for a telescope, as also did Metius, a professor of mathematics, but in the former instance, at least, it was refused, as the apparatus was already known. It seems certain that the instrument was known more or less about Europe, but the honor of its invention is usually given to Galileo, who was the first to describe the instrument and exhibit it in a com- plete form (May, 1609). Galileo having heard that a device to magnify distant objects had been constructed in France or Holland, immediatelv distant object which can be received on a screen, as in the camera obscura (q.v.). If instead of the screen we use a simple microscope or lens to observe this image which is formed in the air, we have an astronomical telescope. This will ap- pear from the accompanying diagram. C represents a convex lens, the object glass of a telescope ; A is a distant object and B is a real and inverted image formed by the lens. The size of this image depends upon the ratio of the two conjugate foci or distances from the lens to the image and the object. As the distance of the lens from the object is necessarily fixed, depend- ing of course on the positions of the object and the observer, in order to obtain as large an im- age as possible, it is necessary to increase the focal length of the lens. From the discussion of lenses in the article on Light (q.v.) it will appear that in order to obtain a large focal length for a telescope an object glass with large radii of curvature is essential. This, however, is not feasible in actual practice, as was found by the early astronomers after experiences with telescopes having focal lengths as great as 600 feet; and lenses of shorter focal length but of improved construction were adopted. The next question involved is the size of the object glass, or, as it is technically expressed, its aperture. The larger the lens the greater will be the amount of light transmitted. The pupil of the eye through which the light producing an im- age passes normally is about 1.5 of an inch in diameter, consequently as much more light will pass through the telescope as the square of the diameter of the object glass is times greater than the square of the diameter of the pupil. But if the image formed is too large, then the light will be distributed and the image will lack brilliancy and not be plainly visible. Taking these facts into consideration, it would appear