ASTRONOMY. 



[KEKLK<-TINU TELESCOPES. 



__ are mirror at a small angle to the object observed, 

 bj which mean* the rays of light proceeding from the 



*. in. 



t-0 



object A B (Fig. 172) were reflected to 6 a, and the 

 image thui formed could be examined iu the ordinary 

 >. 171. 



manner by the eye-piece, the back of the observer being 

 turned to the object This disposition (termed by 

 Hrrschel front view) is, however, only suited for instru- 

 menta with large apertures, in which the loss of light ia 

 less than when reflected from a second mirror. _ 



The great forty-feet telescope of Herechel, with a 

 mirror of four feet in diameter, wan mostly used in this 

 manner, the observer being placed iu a gallery at the 

 end. The engraving below gives a view of this magnifi- 

 cent instrument, which was moved upwards and down- 

 wards by means of numerous ropes and pulleys, whilst 

 the motion to the right and left was facilitated by rollers. 

 With this mighty tube Herechel was able to apply a 

 magnifying power of six thousand ! Unfortunately it 

 was too much exposed to the weather ; the polished 

 mirror was dimmed in one night by the damp atmo- 

 sphere ; and the instrument remained in use fur only a 

 very few years, when it was found unfit for service. 



Fi. 173. 



MKNTS FOR MEASURING run AXOLE. Hitherto 

 our attention ha* been directed to instruments of great 

 magnifying power, intended to bring dmtant objects 

 apparently nearer to the observer. We have now to 

 peak of graduated iiixtruinents of more delicate con- 

 struction, by which the antrnin.iiiitr is enabled to mea- 

 sure, with wonderful accuracy, the size and distance of 

 celestial objects. The true solar time in also obtained, 

 by comparing their angular distances from oil.. 



known objects ; in other words, by measuring their 

 angle. 



The length of the arc of a circle is described be- 

 tween the two sides of an angle, valued by means of a 

 particular arc taken as unity. This arc, by common 

 _& consent, is taken as the three hundred and sixtieth 

 B part of the entire circumference, which is called a 

 degree, marked thus, 1; and the angle which 

 corresponds with it is the angle of a degree (Fig. 

 174). 



It rarely happens, however, that the angle we seek to 

 ] determine comes out exactly in degrees. It becomes 

 necessary, therefore, to divide the degrees into frac- 

 tional parts ; and for this purpose the degree is 

 A divided into sixty parts, called minutes, written thus, 

 B 1'. For further accuracy the minute is again sub- 

 divided into sixty parts, called seconds, written 

 thus, 1". When the instruments which we are 

 about to describe ore properly arranged, and the 

 two telescopes are presented to their proper side of 

 the object whose position or magnitude is to be mea- 

 sured, the angle is read off the graduated scale, 

 Fig. 174. 



which surrounds the circumference of the circle, by 

 means of an index, which extends its limb over the 

 whole length of the arc. 



In measuring the angle by the visual rays which abut 

 on each side of an object, two operations are required. 

 The two rays of the graduated circle must coincide with 

 the two sides of the angle. This is effected by viewing 

 it successively in the direction of each of its sides ; the 

 object being to value the degrees, minutes, and seconds 

 contained in the arc of a circle comprised between the 

 two rays. 



TUB REPEATING CIRCLE consists of a graduated circle 

 A A (Fig. 175) divided into 300, with their 

 respective fractional parts, and fitted with two 

 telescopes furnished with micrometers, the 

 telescopes varying in power according to the 

 size and value of the instrument, and the 

 whole mounted on a pedestal, as represented 

 in the figure. It ia also so arranged as to 

 permit of its being turned iu any required 

 direction. The circle, A A, turns on its own 

 plane round an axis implanted in it perpen- 

 dicularly, and in its centre. This axis tr.c 

 a socket, B, which is fixed to the horizontal 

 axis, C, and is terminated by the weighted 

 drum of the pedestal D. This weighted drum 

 is intended as a counterpoise to the circle 

 and its telescopes, and to prevent it from 

 swinging while turning on the axis C ; the 

 extremities of the axis C are supported by 

 the mounted frame , which is arranged so 

 as to turn freely in the openings left by the 

 mounting. In short, the pedestal, F, can 

 itself turn, with all it carries, round an axis 

 which penetrates its interior, and which is 

 fixed in the foot of the instrument By this 

 disposition of its parts iu turning the circle 

 round the axis C, and at the same time turn- 

 ing all the instrument round the axis of the 

 pedestal E, the plane of the circle can be 

 trained in any desired direction. 



A telescope, S S, is fixed on the upper 

 face of the circle in the direction of one 

 of its diameters, and turns freely round 

 its centre. A second glass, I I, is adapted, 

 in the same manner, to the tinder-face of the circle at an 

 angle with the other, but so adapted as to turn fr. , Iy 

 and independently of the circle. The eccentricity of the 

 under telescope, and the great distance of the objects 

 usually under observation, render the errors scarcely 



i able. 



In Ki^'. I7<; mi fill :u ;<<! portion of the circle ia repre- 

 sented, showing the graduated scale and the extremity of 

 the piece K (Fij 17o). The 1'iece a (Fig. 170) 



