THE TELESCOPE. 
478 
is that made by M. Porro, and which still remains at Paris. 
The object-glass is 54 centimetres in diameter (2P26 inches) 
and has the immense focal length of 15 metres, or nearly 50 
feet. It is mounted with an azimuth and altitude motion, and 
at first sight looks like a gigantic fire-escape. The tube is 
composed of three hundred pieces of wood joined together by 
three thousand screws. Great controversy has arisen respecting 
the merits of this instrument ; but M. Bulard, the astronomer 
at Algiers, asserts that he could see everything with it that he 
had with Earl Posse’s 6 feet mirror ; and M. Porro states that 
he has detected two new stars within the trapezium of Orion. 
It is certain, however, that although in existence for six or 
eight years, it has been very little used. Comparing this 
immense instrument with the modest five-feet telescope of M. 
Goldschmidt, erected on a moveable tripod stand in his g’arden 
near Paris, and with which he has discovered so many planets 
and variable stars and nebulae, and made so many curious ob- 
servations, it will be seen what may be done with care and 
patience even with small means. 
Should the observer be provided with a telescope 
mounted equatorially, he has simply to set the declination and 
hour circles to the object required, and of which he has of 
course the right ascension and polar distance given. In other 
cases, in searching for any particular object, he must make use 
of an ordinary star-map divided in the usual men nor into right 
ascensions and declinations, on which having marked the 
position of the required nebula, comet, or double star, he 
guides the direction of the telescope by means of the bright 
neighbouring stars of the constellation in which the object is 
placed ; and with a little management this operation will soon 
become easy. In the same way, to find any particular planet, 
its place on the required day will be taken from the “Nautical 
Almanac,” or any similar work. Great assistance in the choice 
of stellar objects will be derived from Admiral Smyth’s “Celes- 
tial Cycle,” in which the most prominent clusters, nebulae, 
double and variable stars are fully described, and full parti- 
culars given of their positions, brightness, and difficulty. In 
pointing the telescope, the observer makes use of the small 
side tube, termed the finder, which, giving a large field of view, 
helps him in the alignement of the faintest stars marked on the 
map. In searching for comets, the lowest power of the tele- 
scope should be made use of; and if a nebulous object be 
caught sight of, its place should be transferred to the map, in 
order to determine whether any fixed body of that character 
has been before observed. In all cases it is almost useless to 
search for objects with high powers, which not only gives a 
small field but renders them very faint. It will be found that 
