PHYSICAL ASTRONOMY 155 
marked + are deceased Baumann (in Stuttgart); Breithaupt (in Cassel) ; 
Cauchoix (in Paris) ; Dollond+- (in London) ; Douve+ (in Berlin); Emery-+, 
Ertel (in Munich) ; Fraunhofer+ (in Munich); W. Herschel+, Jiirgensen 
(in Copenhagen); Kessels (in Altona); Lerebours (in Paris); Merz and 
Mahler (in Munich); Oertling and Pistor (in Berlin); Pléssl (in Vienna) ; 
Ramsden+ (in London) ; Repsold, sen.,+ and Repsold Bro’s (in Hamburg) ; 
Reichenbach+ (in Munich); Lord Rosse (in Ireland); Troughton+ (in 
London); Voigtlander (in Vienna) ; and others. 
THE TELESCOPE; ITS DIFFERENT CONSTRUCTIONS. 
The Dorpat Refractor. 
81. The Telescope is that optical instrument which represents distant 
objects more distinctly and of larger size than they appear to the naked eye. 
There are two principal kinds of telescopes, dioptric or refractors, and 
catoptric (or more properly catadioptric) or reflectors. In refractors, 
observations are conducted by means of glasses alone, as, for example, the 
eye glass and object glass; in reflectors a concave mirror is used instead 
of the object glass. Both were invented in the sixteenth and seventeenth 
centuries ; the refractor in 1590, by Jansen of Middleburg, and the reflector 
in 1644, by Mersenne. 
Refractors consist of a cylindrical tube at whose two extremities are 
placed the lenses employed. The lens receiving the rays of light from the 
object, is called the object glass or objective. The one through which the 
image produced is seen by the eye is called the eye-glass or ocular. 
Kye-glasses are stmple when but one lens is used, compound when several 
lenses are combined together. Besides the magnifying power of the tele- 
scope, reference must be had to the diameter of the field of vision, the illu- 
mination or amount of light, and the degree of distinctness of vision. 
There are three kinds of refracting telescopes ; the astronomical, invented 
by Kepler ; the terrestrial, invented by De Rheita, 1665; and the Galilean. 
The astronomical telescope is formed by the combination of two convex 
lenses; the one, the eye-glass, can be made to approach to or recede from 
the object glass. This form of telescope, although it represents objects 
inverted, exhibits them very clearly and much magnified; having also a 
large field, it is on this account principally used by astronomers. 
The second kind, or the terrestrial telescope, is a combination of four 
convex lenses ; three of them, fixed immovably in one tube, can be made to 
vary their position with respect to the fourth—the object glass. This 
telescope, which exhibits objects erect, may be considered as a combination 
of two astronomical telescopes, of which the one represents the inverted 
image of the other again inverted, consequently actually erect. Neverthe- 
less, it is more advantageous to furnish the terrestrial telescope with four 
eye-glasses, as is generally done. The eye-glass nearest the eye thus 
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