158 | ASTRONOMY. 
this refractor leaves all known reflecting telescopes far behind. »-Struve saw 
the multiple star + Orion certainly 16-fold, while Schroter with his 25 foot 
refractor only saw it 12-fold; the little companion of the star 8 Orion (Rigel) 
was seen through the Dorpat refractor very distinctly just before sunset ; 
and even w? Leonis, one of the most difficult double stars, in this instru- 
ment, was recognised without any difficulty as a double star. - In con- 
clusion, it remains to be mentioned, that this telescope is placed in a building 
specially arranged for rotation. The cost of the instrument amounted to 
10,500 florins ($5000). 
More recently the observatory at Pulkowa has received a somewhat 
larger refractor from Munich, whose object glass has an aperture of )14;% 
Paris inches. The new: Berlin observatory also possesses a large refractor 
of remarkable excellence from this same celebrated manufactory~ at 
Munich. The observatories of Cambridge, Washington, and Cincinnati, 
likewise possess refractors of great power. 
When in use, the telescope is directed to the object to be investigated, 
whose motion is followed by changing the position of the instrument. If 
the telescope be arranged as in the Dorpat refractor, all the clamps.are 
to be loosened, the object found with the seeker, and the instrument then 
fixed ; upon which, by means of the clock-work, it is moved in such a man- 
ner as always to have the object in the field of vision. | 
The Reflecting Telescope ; Herschel’s Giant Telescope. 
82. Catoptric (catadioptric) telescopes, commonly called reflectors, form 
the second grand division of these instruments. 
~The reflector is a telescope which, instead of an object glass, has two 
mirrors, an objective and a reflecting mirror. Newton constructed. the 
first reflecting telescope of the form now bearing his name. This consists 
of a hollow cylinder, so placed upon a frame as to be readily directed to 
any point of the heavens. The one end of this cylinder is closed by a 
spherically concave metallic: mirror or speculum, whose focus lies in the 
common axis of the cylinder and mirror. At a little distance from the 
focus of the speculum is placed a plane mirror inclined to the axis of the 
cylinder at an angle of 45°. A beam of light impinging upon the concave 
mirror, is reflected in a cone upon the small plane mirror, and thence into 
the ocular placed in the side of the instrument and at right angles to it. 
This arrangement, therefore, represents objects inverted, unless, as in the 
terrestrial telescope, erecting lenses be placed in the eye-piece. After 
Newton’s death another form, the Gregorian, with the smaller reflector 
concave, came into use; this represented objects erect. A third kind, the 
Cassegrainian, was also introduced. This last has the small anterior 
reflector convex instead of plane, as in the Newtonian, or of concave, as 
in the Gregorian; objects are represented in it inverted. 
Herschel improved the Newtonian telescope by omitting the small 
reflector, fixing the large concave speculum at a slight angle to the axis. 
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