94 Dr Searle, Experiments with a plane diffraction grating 



plate, the faces of the plate are made parallel to the shaft and the 

 axis of the colUmator is made perpendicular to the shaft. In this 

 case the image of the collimator wires can, by turning the circle, be 

 made to coincide with those wires, when either side of the plate 

 faces the colhmator. If the plate has been suitably placed, it will 

 be possible, by turning the goniometer on its shaft, to receive the 

 image of the collimator wires on the focal plane of the goniometer. 

 The "vertical" cross-wire of the goniometer, i.e. the wire perpen- 

 dicular to the shaft, is now adjusted so that it coincides with the 

 image of the corresponding wire of the colhmator. The line of 

 colhmation is then perpendicular to the shaft. The goniometer is 

 then put into position and its shaft is levelled. 



The axes of the collimator, of the grating shaft and of the 

 goniometer shaft are adjusted to be approximately in the same 

 horizontal plane. The plane of the grating is made vertical, and 

 the goniometer stand is adjusted in azimuth so that one of the 

 diffracted images of the collimator wires can be made to coincide 

 with the goniometer wires by turning the goniometer on its shaft. 



When the adjustments already described have been effected,, 

 and when the plane of the grating G is vertical, the diffracted beams 

 are parallel to the plane OTN. If OT is inclined at an angle 8 to 

 the grating shaft, the direction of OT will be changed by 28 if 

 G is turned through 180° about the axis of the shaft from Position 1 

 to Position 2, when the plane is again vertical. The goniometer is 

 turned to receive a diffracted beam when G is in Position 1. If, 

 when G is turned into Position 2, the inchnation of the beam to 

 a horizontal plane is changed, the grating must be turned in its 

 own plane until the inclination is the same for both positions. 



Since ijj is always small, cos i/j is nearly unity and hence, by 

 (22), sin oj has a nearly constant value, for Wg is independent of 6. 

 Hence, if the image of C, the intersection of the collimator wires, 

 lies on D, the intersection of the goniometer wires, when the plane 

 of G is vertical, the image of C will always lie very near the 

 "vertical" cross-wire, and one setting of the goniometer stand will 

 suffice for all values of 6. 



The plane of the grating is made horizontal and the index 

 reading is taken. It is then turned through 90°; it is now vertical 

 and in its zero position. The goniometer is next adjusted so that 

 the image of C lies on the horizontal wire of K. The goniometer is 

 then in its zero position. 



The grating is now turned through 10°, 20°, ... on sither side 

 of the zero, and the goniometer is turned on its shaft to bring the 

 image of C on to the horizontal wire of K in each case. If the 

 grating circle has tivo indices, the grating is turned through 10°, 

 20°, ... as indicated by one index. In reducing the observations the 

 mean of the angles furnished by both indices is used. 



