MIRROR METHOD. 



horizontal divided scale, EE', is placed at a certain distance from the 

 mirror, below the horizontal MN, and perpendicular to its direction. 

 A little above the scale is a telescope, L, movable about a horizontal 

 axis, so that its optical axis describes the plane CC'DD'. The 

 telescope is directed on the mirror, and it is adjusted so as to give a 

 sharp image of the scale as seen by reflection. The conditions of 

 adjustment will be satisfied for the telescope, if, by a simple turn of 

 the eye-piece and a rotation about the horizontal axis, the wire by 

 which the mirror is suspended, and the image of a plumb-line, DD', 

 stretched in front of the middle of the object-glass formed in this 

 mirror, are made to coincide successively with the cross wires ; the 

 scale is in adjustment if its two ends, E and E' are at the same 

 distance from the mirror, and if during the oscillations the image of 

 these two ends is at the same height in the plane of the network. 



The position of the mirror is defined by the division of the scale 

 which makes its image on the cross wires of the telescope. If the 



Fig. 129. 



adjustment is perfect, the zero of the division corresponds to that 

 division of the scale which in the telescope is concealed by the 

 plumb-line DD'. 



The divisions of the scale may be numbered on each side, 

 counting from the centre ; but in order to avoid the readings of 

 contrary signs and of change of directions, it is better to place the 

 zero of graduation at one end of the scale. 



663. CALCULATION OF THE DEFLECTION. Let us take as plane 

 of figure the horizontal plane which passes through the centre of the 

 mirror (Fig. 129). Let C be the axis of rotation ; let the lines CO, 

 CN, and CX, representing the vertical planes which pass respectively 

 through the optical axis of the telescope, through the actual position 

 of the perpendicular to the mirror, and through that division of the 

 scale the image of which coincides with the cross wires. The two 



