CONCERNING SCIENTIFIC APPARATUS. 515 



is called the error of Parallax. In some instruments, therefore, the observation 

 is made through an eye-hole in a definite position. A better plan, however, is 

 to place a plane mirror under the needle, and in taking the reading to place 

 the eye so that the needle appears to cover its own reflexion in the mirror. 



15. SPIEGEL- ABLESUNG, OR MIRROR-READING. 



A still more accurate method is that invented by Poggendorff, and used 

 by Gauss and Weber in their magnetic observations. A small plane mirror is 

 attached to the indicating piece, so as to 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. 



16. 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 



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