TELESCOPE. 



547 



axvriu, to see) ; an optical instrument, employed 

 for viewing distant objects, by increasing the ap- 

 parent angle under which they are seen without its 

 assistance, whence the effect on the mind of an 

 increase in size, or, as commonly termed, a magni- 

 fied representation. (See Optics.') The telescope 

 is perhaps one of the most important inventions of 

 science, as it unfolds to our view the wonders of 

 the heavens, and enables us to obtain the data for 

 astronomical and nautical purposes. As the use of 

 the instrument depends upon the proportionate dis- 

 tance of the glasses, and this distance requires to be 

 changed to suit the nearness or remoteness of the ob- 

 ject, and the vision of the observer, the tube of the 

 telescope is so contrived as to admit of being length- 

 ened and shortened, according to circumstances. 

 The invention of the telescope is ascribed to differ- 

 ent persons, among whom are John Baptista Porta, 

 Jansen of Middleburg, and Galilei. The time of its 

 first construction is considered to have been about 

 1590 ; but, in 1608 and 1609, we find these instru- 

 ments for sale at very high prices by Dutch opticians ; 

 and in the latter year, Galilei constructed one with- 

 out having seen those of the Dutch, by fitting a pla- 

 no-convex and a plano-concave lens in a tube of lead. 

 The simplest construction of the telescope consists 

 merely of two convex lenses, so combined as to in- 

 crease the apparent angle under which the object is 

 seen. The lenses are so placed that the distance 

 between them may be equal to the sum of their 

 focal distances. The lens nearest the eye is called 

 the eye-glass, and that at the other extremity of the 

 tube the object-glass. Objects seen through this 

 telescope are inverted, and on that account it is in- 

 applicable to land observation ; but at sea it is oc- 

 casionally used at night and in hazy weather, when 

 there is little light, and is, therefore, sometimes 

 called the night telescope. The astronomical tele- 

 scope is constructed in this manner, as the inversion 

 of the object is immaterial in celestial observations. 

 The common day telescope, or spy-glass, is an in- 

 strument of the same sort, with the addition of two, 

 AT even three or four other glasses, for the purpose 

 of presenting the object in an erect position, in- 

 creasing the field of vision, and diminishing the 

 aberration caused by the dissipation of the rays. 

 But the aberration and chromatic error of tele T 

 scopes were not completely obviated until the 

 invention of the reflecting and achromatic telescopes, 

 which, when accurately constructed, present the 

 object to the vision free from all distortion or 

 chromatic dispersion. The reflecting telescope was 

 invented by father Mersenne, a Frenchman, in the 

 middle of the seventeenth century. Concave mir- 

 rors have the property of uniting the rays of light 

 which proceed from any object, so as to form an 

 image of that object at a certain point before the 

 mirror. (See Mirrors.) If the distance of the 

 object is so great, that the rays proceeding from it 

 strike upon the mirror parallel to each other (which 

 is the case with the heavenly bodies), the distance 

 of the image is equal to half the radius of the 

 sphere, of which the mirror is an arch, and the 

 point where it is formed is called the focus of the 

 mirror. (See Burning Mirror.) This property 

 of the concave mirror has caused it to be used in 

 the observation of the heavenly bodies; and the 

 instrument constructed with such a mirror, is called 

 a reflecting telescope. The simplest constructions 

 of this kind were those in which the image, formed 

 in the focus of the mirror, was used directly, and a 

 convex eye-glass was employed to magnify the 



angle under which it was seen ; and this, in fact, 

 still continues to be the principle on which reflect- 

 ing telescopes are constructed. But as this con- 

 struction is attended with some difficulties in prac- 

 tice, Newton, and, since him, Cassegrain, Gregory, 

 Hadley, Short, and the Herschels, have introduced 

 some modifications in it. Newton, by means of a 

 second reflection from a plane mirror, inclined at a 

 certain angle, threw the image of the object into 

 such a position in the tube of the telescope, that 

 it could be easily examined from the side of the 

 tube, through a plano-convex eye-glass, in whose 

 focus it was situated. In the Gregorian telescope 

 there is a large mirror with a small hole in its 

 centre ; opposite to this is placed a second small 

 mirror in the axis of the larger one, and at a distance 

 from it a little more than the sum of their focal 

 distances. By means of this construction the image 

 formed by this double reflection is viewed through 

 one or more eye-glasses, fixed in the direction of 

 the opening, and, therefore, the observer is sta- 

 tioned in a line with the object ; while, in the 

 Newtonian telescope, he is at right angles to it. 

 The Cassegrainian is constructed in the same way 

 as the Gregorian, with the exception of having a 

 small convex instead of a concave speculum. Her- 

 schel gave the mirror such a position that its focus 

 should fall directly under the edge of the upper 

 aperture, so that the observer, in vie wing the image, 

 should not intercept the light : this he called a 

 front-view telescope. It is plain that the size of the 

 mirror, and, consequently, its focal distance, have 

 an effect upon the magnitude of the image ; and 

 modern astronomers have, therefore, employed 

 some instruments of this kind of great bulk. Her- 

 schel's gigantic telescope, erected at Slough, near 

 Windsor, was completed August 28, 1789 : and on 

 the same day the sixth satellite of Saturn was dis- 

 covered. The diameter of the polished surface of 

 the speculum was forty-eight inches, and its focal 

 distance forty feet. It weighed 2118 pounds, and 

 was placed in one end of an iron tube four feet ten 

 inches in diameter. The other end was elevated 

 towards the object, and had attached to it an eye- 

 glass, in the focus of the speculum, as above men- 

 tioned. The observer was mounted in a gallery, 

 movable with the instrument, and having his back 

 to the object. The light obtained from so large a 

 surface was truly surprising, and enabled objects, 

 otherwise invisible, to become extremely interest- 

 ing. (A full description of this instrument, illus- 

 trated with eighteen plates, may be found in the 

 Transactions of the Royal Society for 1795.) The 

 frame of this instrument having become much 

 decayed, it has been taken down, and another, of 

 twenty feet focus and eighteen inches diameter 

 erected in its place (1822), by the distinguished Sir 

 J. F. W. Herschel, son of Sir William. The largest 

 front-view telescope, at present in England, is that 

 erected at the royal observatory at Greenwich, by 

 Mr Ramage, in 1820. The diameter of the re- 

 flector is fifteen feet, and its focus is twenty-five 

 feet. Schroter had an excellent telescope of this 

 kind at Lilienthal, of twenty-five feet focus, by 

 which the Milky Way was separated into innumer- 

 able small stars. Schrader, at Kiel, had a similar 

 instrument of twenty-five feet focus, at the close 

 of the last century. Another improvement has 

 been recently introduced in the reflecting telescope, 

 i by making the speculum of platina, so that it will 

 not suffer from rust. Having noticed some of the 

 most valuable modifications of the refecting tele- 

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