TELESCOPE. An optical instrument for viewing distant objects. This sub ject belongs more properly to the depart ment of science, but as it is an instru ment in such general use a short notice will be given here. It is not the mere distance which renders an object invisible or even indistinctly seen. TI ere are other causes which come into play, such as the diminution of the angle which the object subtends which diminishes as the distance increases: again, the light which renders the object visible, becomes less dense as the distance increases, but in a much faster proportion, and all the rays of light which have an object do not reach the observer, some being lost in the air in their passage. For an object to be seen in ordinary daylight it mast subtend at the eye an angle of 30". The least angle under which contiguous objects may be clearly distinguished is one minute : by the aid of the telescope an enlarged image of the object is obtained, and within cer tain limits the object is not only appa rently enlarged but rendered brighter to the eye. The telescope, then, enlarges the angle under which objects are seen. Its invention has been ascribed to vari ous persons. Brewster believes that either Roger Bacon or Baptista Porte formed it for experiment. It has also been ascribed to Metius, Lippersey, and Jansen. Lippersey in 1608 actually made one, but Jansen's instrument being more notorious roused the attention of Galileo, in 1609, who set about considering the means whereby distant objects might be rendered visible. He was soon in posses sion of a telescope which magnified three times : subsequently he increased its power very much, and at the close of that yea rhe discovered the satellites of Jupiter. There are two kinds of telescopes, re fracting and re,tlectissg telescopes. The former depending on the use of appro priately figured lenses, through which the rays of light are passed, and the lat ter on the use of specula or polished me tal mirrors which reflect the rays ; an inverted image of the object being form ed in both cases in the focus of the lens or mirror. These are a later invention than refracting telescopes, were of a simple character at first, made up chief ly of e lens, forming the object-glass, and an eye-glass also of one lens, but of a much shorter focus. The different re frangibility of the luminous rays pro duced a series of prismatic colors, which tinged the images formed by the tele scope and rendered their outline thereby indistinct.
Under the article LENS has been men tioned the various forms of these sur faces, to which reference may be had for illustration in describing this instrument. By combining these in a tube, or case, the parallel rays from the object are brought to one point without loss or interference. The naked eye can see objects distinctly when placed at a great distance, that is, when the rays proceeding from the ob ject are parallel or nearly so ; consequent ly if an object be placed very near the eye, and it the rays which flow from it can be made to enter the eye nearly pa rallel to each other, we must see it dis tinctly. This parallelism may be effected by placing close to the eye a convex lens and holding the object in its focus. If the latter be called F, and the centre of the lens C, by placing the object a little near er than F the rays which flow from it may receive the exact degree of divergen cy which they have when the object is placed six inches from the eye, the near est distance at which we can see minute objects distinctly. If the distance C F be one inch, the object at F will have its apparent magnitude six times greater than when it is seen at the distance of six inches without the lens. It is therefore said to be magnified six times by the lens. This constitutes the single micro scope, and the magnifying power of such may be always found by dividing six inches by the focal distance of the lens. Thus, a lens the 1-10th of an inch in focal length will magnify 60 times, and one the 1-1000th of an inch 600 times. To the instrument with one lens which thus magnifies where the naked eye is six inches behind it, additional magnifying power may be given by bringing the eve within all inch of the image, that is, by viewing the image with an additional lens when the focal distance is an inch. This lens will magnify the image six times, and if that image had been magnified bv the former lens ten times, then the mag nifying effect of the two lenses will be 10 X 6 = 60 times. Such is the astrono mical telescope by which objects are seen inverted, and the magnifying power of which is always equal to the focal length of the oldect-glass, or the lens next the object, divided by the focal length of the eye-glass or the lens next the eye.