REFRACTING TELESCOPES.] 



ASTRONOMY. 



993 



be applied, and with better effect in one of 10 feet in 

 length, than to those formerly made, which were some 

 hundreds of feet in focal distance. 



It is a matter of considerable moment to have the 

 telescope mounted as steadily as possible, and to be able 

 to direct it easily to any particular part of the heavens, 

 so as to follow any object with the requisite facility. In 

 the following figure (Fig. 168) a telescope-stand is repre- 



Fig IBS, 



sented, which serves these useful purposes. It consists 

 of a firm support, A, resting on the ground by 

 means of the feet B' B' B' and the rollers B B B, 

 and two movable supports, C and 1), the former 

 of which directly supports the telescope E, and is 

 connected with the stand A by means of hinges 

 at M M, by which it can be inclined more or less 

 to the horizon. The second support, D, ia con- 

 nected with the first in a similar manner by the 

 hinge n, by which means the angle between the 

 two supports, C and D, may be varied at will. 

 The inferior beam, o o, of the branch D, should 

 Klide along the inclined side of the stand A, 

 :n:<l this motion produces the requisite elevation 

 or depression of the extremity n, which turns 

 fin the hinge m. Two endless chains, q q, pass round 

 the stand A, and are attached to the ends o o of the branch 

 D. An axis, r, terminated by i wo handles, s t, 

 carries a pinion which works in a wheel mounted 

 on a second axis, <; this second axis is furnished 

 with two pinions at both ends, of which the 

 t< eth catch in the links of the endless chain 

 qq. In turning round the handle , the chain 

 i ; thus worked up and down ; the portion o o 

 of the support D slides on the inclined plane, p p, 

 and the projecting branch is raised more or less. 

 The telescope, E, rests in u u, the latter part being con- 

 VUL. i. 



nected with the support C by a pin at the object end, 

 round which it can easily be turned as on a pivot. At 

 the eye end of the telescope, the support u u rests upon 

 two wheels, and a milled head, V, attached to a pinion, 

 works in the toothed edge of the support C in such a 

 manner, that in turning the milled head it turns the tele- 

 scope to the right or left. The observer, by turning one 

 of the handles, ss, with the one hand, and the milled 

 head, V, with the other, is thus able to change 

 the direction of the instrument according to 

 his wish. A smaller telescope, Z, is commonly 

 attached to the tube of the larger one, and is 

 known by the name of the finder. Having a 

 larger field of view than the principal telescope, 

 the observer can discover the object he is in 

 search of with greater facility ; and by placing it 

 in the centre of the finder, he guides the larger 

 telescope directly upon it. 



REFLECTING TELESCOPES. If an object A B 

 be placed before a concave reflecting mirror M 

 (Fig. 169), a similar effect is produced as when 

 the rays of light proceeding from such an object 

 pass through a convex lens. The ray of light 

 passing from the point A will be reflected to the 

 point a, while those passing from B will take 

 the direction 6, and the inverted image a 6 will 

 be produced. The image a 6 can be magnified in 

 tlie same manner as the image formed by a convex 

 lens, though evidently not with the same facility ; 

 for here the image is formed between the object 

 nnd the mirror, and there is a difficulty in know- 

 ing in what manner the eye-piece is to be placed. 

 To obviate this difficulty, the inventor of this 

 telescope, the celebrated Gregory, thought of 

 placing a smaller concave lens N (Fig. 170) 

 which would only intercept a portion of the 

 rays proceeding from any object after their 

 passage through the image a 6, and to send them 

 through the opening at the centre of the mirror 

 M, in such a manner as to produce a second 

 image, a' 6', which could be examined with the 

 requisite distinctness by the eye-piece o. By this 

 combination an erect image would be produced, 

 and the telescope would be used in precisely the 

 same manner as the Galilean telescope. The 

 telescope independently invented a few years 

 later by Sir Isaac Newton, is more simple than 

 the former, receiving the rays which pass from 

 the great mirror on a plane reflector N (Fig. 171), 

 inclined at an angle of forty-five degrees, 

 before they arrive ut the focus a 6, and pro- 

 duce the image o' 6', which can be examined 

 by means of the eye-piece O. In thia form of the 

 Fig. 109. 



reflecting telescope, the observer looks in a direction 

 perpendicular to that of the object observed. In order 

 Fig. 170. 



to obtain as much light as possible, a considerable portion 

 of which was lost by being reflected from successive sur- 

 faces, Ilerschel conceived the plan of inclining the great 



C i> 



