THE TELEGRAPH. B^^ 



are not necessary for astronomical telescopes ; for no incon- 

 venience arises from seeing the celestial bodies inverted. 



When very great magnifying power is required, telescopes 

 are constructed with concave mirrors, and called reflecting 

 telescopes. Mirrors are used in order to bring the image 

 nearer the eye ; and a lens or eye-glass is for the same pur- 

 pose as in the refracting telescope, that is, to magnify the 

 image. The Newtonian reflecting telescope consists of a 

 tube, towards the end of which a concave mirror is placed. 

 The reflected converging rays, before they reach the focus, 

 are made to fall upon a plane mirror placed at an angle of 

 forty-five degrees, and thus are thrown upwards to the focus 

 of a convex lens fixed in the upper side of the telescope, 

 through which the eye looks down on the image. In the 

 telescopes made by Dr. Herschel there is but one mirror, 

 which is placed at the lower end of the tube, with such 

 an inclination, that the rays are brought to a focus and the 

 image formed near the edge of the upper end of the tube. 

 The image, therefore, is formed by only one rcjiection, and its 

 brightness, when viewed through the lens is, on this account, 

 greater than that in the Newtonian telescope. The head 

 of the observer, when a large aperture is wanted, may be 

 placed entirely at one edge of the tube, so as not to intercept 

 any of the rays at the time of making an observation ; but 

 as the eye looks down the tube, the back must be turned to 

 the object. Dr. Herschel's grand telescope is nearly forty 

 feet long, and four feet ten inches in diameter. The con- 

 cave polished surface of the great mirror is forty-eight inches 

 in diameter, and it magnifies six thousand times. This noble 

 instrument was, in all its parts, constructed under the sole 

 direction of Dr. Herschel : it was begun in the year 1785, 

 and completed August 28th, 1789, on which day was dis- 

 covered the sixth satellite of Saturn. 



The telegraph is a machine for communicating intelli- 

 gence at a considerable distance, by making various signals, 

 — which have been previously agreed upon between two 

 parties, — to represent letters, words, or ideas. No machine 

 for making signals can with propriety be called a telegraph, 

 unless it is adapted to express a suflicient number of letters 

 or words to form a complete language, and which can be 

 made, therefore, to communicate any information which can 

 be expressed by oral or written language. Less perfect sys- 



