A. MATHEMATICS AND ASTRONOMY. 39 



resenting Coggia's comet of 1874, a chart of the Pleiades, 

 and a lithographic plate representing the phenomena of the 

 lunar eclipse of the 1st of July, 1863. In the latter the va- 

 ried colorings of the portions of the disk of the moon are 

 very striking. In the chart of the Pleiades, the nebula 

 around Merope is represented as very distinct. There seems 

 to be good reason to doubt the variability of this object. 

 The series of drawings of the comet of 1874 appears to be 

 remarkably complete, and is very skillfully executed. 



THE GREAT PARIS TELESCOPE. 



The great telescope at Paris is now quite complete, ex- 

 cept in a few minor matters. It was officially exhibited on 

 the 7th and 9th of October. The mirror is 47 inches in 

 diameter, with a focal distance of 23 feet. The weight of 

 the mirroris half a ton; it is constructed of glass, and is sil- 

 vered by the Foucault process. A metallic mirror of the 

 same size would probably weigh four tons. The cost of the 

 mirror was 2000. The telescope is mounted like an ordi- 

 nary equatorial refractor. A magnifying power of 500 has 

 been found to answer excellently for observing minute stars. 

 The finder, together with the ocular and the small plane mir- 

 ror, are all placed at the front of the tube, and can be rotated 

 around its axis, thus facilitating the use of the large iron 

 winding staircase from which observations may be made. 

 The height of this staircase is about 40 feet, and its weight 

 six tons. It rests on two circular iron rollers, and is always 

 placed on the same side of the tube as the counterpoise. It 

 is moved by special machinery. The observations are made 

 in the open air, but the instrument is protected at other 

 times by means of an iron house, which building weighs 

 twelve tons, and is movable on rollers by means of special 

 machinery, so that it can be rolled to one side, and in less 

 than a quarter of an hour the huge telescope can be uncov- 

 ered and pointed upon any object, however minute. The 

 entire cost of the telescope and appurtenances has been 

 8000, and it has occupied six years in building, the work 

 having been interrupted several times by the events of the 

 civil war and other matters; its final accomplishment is 

 due to Le Verrier's perseverance. This distinguished as- 

 tronomer now contemplates undertaking a large refractor, 



