COMMON THINGS TIME. 



ascertaining the moment at which an object is precisely J 

 way between the point at which it enters and that at which it 

 leaves the field of view, he would know the moment at which 

 it passed the meridian. 



This is accomplished by a very simple and admirable con- 

 trivance. In the eye-piece of the telescope is fixed a small frame , 

 across which are extended vertically five or seven fine wires or 

 filaments at equal distances apart, the centre one passing through 

 the middle of the field of view, and one horizontal wire also 

 passing through the centre, and therefore dividing all the vertical 

 wires equally. 



The field of view and the system of wires are shown in fig. 2, 

 (p. 129), where E w is the horizontal, and IT s the middle vertical 

 wire. It must be observed that the wires are so extremely fine that 

 even when they are magnified by the eye-glass of the telescopo 

 they still appear like mere hairs. The number of vertical wires 

 being always odd, one of them will necessarily pass through the 

 centre. The instrument represented in fig. 1 is provided with 

 such adjustments, that the middle wire N s, can be brought to 

 coincide with the utmost precision with the meridian. 



The magnifying power of the telescope has the same effect upon 

 the apparent motions of objects as upon their apparent magnitude. 

 It increases the one in the same proportion as the other. The 

 consequence is that, although the apparent diurnal motion of 

 celestial objects is no more perceptible to the naked eye than is 

 the motion of the hour-hand of a watch, yet when viewed with 

 the telescope, this motion is very distinctly perceptible. The 

 stars seem like so many luminous insects, creeping with a visible 

 motion across the field in horizontal directions, and passing in 

 succession behind each of the parallel vertical wires. 



27. So rapid is this apparent motion of the celestial objects 

 across the field of the telescope, that a star is- seen to pass from 

 one side to the other of one of the vertical wires between two suc- 

 cessive beats of the clock. Thus it may be seen at o, fig. 2, at 

 the moment marked by one beat, and at o', at the moment marked 

 by the next. Practised observers are in such case able t> 

 determine to the tenth of a second, or even less, the instant of its 

 transit over the wire. Thus if the moment it is at o, be 10 h 20 m 

 20 s , and that at which it is at o', be 10 h 20 m 21% the observer will 

 be able to say for example that the instant at which it has passed 

 the wire N s is more than 10 h 20 m 21 s -4, and less than 10 h 20 m 

 21 8> 5, and he may assign the time as 10 h 20 ra 21 s -45. Different 

 observers acquire, according to their respective aptitudes, different 

 degrees of skill in such observations, and in all cases the results 

 of their observations can be checked by comparing those obtained 

 126 



