GENERAL CONSIDERATIONS. 13 



turn with it about its axis, which we may suppose vertical. A 

 divided scale is placed so as to be perpendicular to this axis, and 

 so that a normal to the scale at its middle point passes through 

 the axis. The image of this scale by reflexion in the mirror is 

 observed by means of a telescope having a vertical wire in the 

 plane of distinct vision. As the indicating piece turns about its 

 axis, the image of the scale passes across the field of view of the 

 telescope, and the coincidence of the image of any division of the 

 scale with the vertical wire of the telescope may be observed. 



The error of parallax is entirely got rid of by this method, for 

 the two optical images whose relative position is observed are in 

 the same plane. 



Another method of using the mirror is to reverse the direction 

 of the rays of light by removing the eye-piece of the telescope, 

 and putting the flame of a lamp in its place. The light emerging 

 from the object-glass falls on the mirror, and is reflected so as to 

 form on the scale a somewhat confused image of the flame, with a 

 distinct image of the vertical wire crossing it. The reading is 

 made by observing the position on the scale of the image of the 

 vertical wire. In many instruments the telescope is dispensed 

 with, and the mirror is a concave one, as in Thomson's reflecting 

 galvanometer. 



Some German writers distinguish this method of using the 

 mirror and scale with a lamp as the objective method, the method 

 in which the observer looks through the telescope being called 

 the subjective method. The objective method is the only one 

 adapted for the photographic registration of the readings. 



1 6. RAMSDEN'S GHOST. 



To ascertain the exact position of an instrument with respect to 

 a plumb-line without touching the line, Ramsden fixed a convex 

 lens to a part of his instrument, and placed a wire so that when 

 the instrument was in its proper position the image of the plumb- 

 line formed by the lens exactly coincided with the fixed wire. By 



