ASTRONOMICAL USES OF THE TELEGRAPH. 



more minute division of that small interval. In order to enable 

 the reader fully to appreciate the benefit which the telegraph 

 has rendered to astronomy, it will be necessary here briefly to 

 explain the manner in which this kind of observation has hitherto 

 been made. 



To determine the moment at which the visual ray proceeding 

 from a celestial object has some definite direction, two things are 

 necessary 1st, to ascertain the direction of such a ray; and, 

 2ndly, to observe the time when it has such direction. The tele- 

 scope, with its accessories, supplies the means of accomplishing 

 the former, and the astronomical dock the latter. 



If T T', fig. 93, represent the tube of a telescope, T the extremity 

 in which the object-glass is fixed, and if the end where the 



Fig. 93. 



5* 



images of distant objects to which the tube is directed are formed, 

 the visual direction of any object will be that of the line s' c 

 drawn from the image of such object formed in the field of view 

 of the telescope to the centre c of the object-glass, for if this line 

 be continued, it will pass through the object s. 



But since the field of view of the telescope is a circular space of 

 definite extent, within which many objects in different directions 

 may at the same time be visible, some expedient is necessary by 

 which one or more fixed points in it may be permanently marked, 

 or by which the entire field may be spaced out as a map is by the 

 lines of latitude and longitude. 



This is accomplished by a system of fibres or wires, so thin that 

 even when magnified they will appear like hairs. These are 

 extended in a frame fixed within the eye-piece of the telescope, so 

 that they appear when seen through the eye-glass like fine lines 

 drawn across the field of view. 



The system consists commonly of five or seven equidistant 

 wires, placed vertically at equal distances, and intersected 

 at their middle points by a horizontal wire, as represented in 

 fig. 94. When the instrument has been adjusted, the middle 

 wire m mf will be in the plane of the meridian, and when an 

 object is seen upon it, such object will be on the celestial meri- 

 dian, and the wire itself may be regarded as a small arc of the 

 meridian rendered visible. 



The eye of the observer is occupied in watching the progress 

 of the object moving over the wires in the field of view of the 



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