Dr Searle, Experiments with a plane diffraction grating 93 



lens is the line of collimation, or axis, of the collimator. The parallel 

 beam defined by C is parallel to this hne. After the light has 

 passed through the grating, it is 



received by a goniometer K, and -^ S 



an image of the collimator wires r nC 



is formed in its focal plane. To 

 fix the line of collimation, cross- 

 wires are placed in the focal 

 plane; they intersect in D. The u'^ 

 goniometer is carried on a hori- 

 zontal revolving shaft B, and its 

 line of colhmation is perpen- 

 dicular to the shaft. One cross- 

 wire is parallel and the other 

 perpendicular to the shaft; the E 

 latter is also perpendicular to the U/i 

 shortest distance from D to the 

 axis of the shaft. The shafts are 

 provided with divided circles E, Fig. 4. 



F, which are read by aid of the 



pointers U, U', V, V. A balance weight W is attached to the circle 

 F. The point of intersection of the hne of colhmation of K with 

 the axis of B should lie approximately on the centre of the grating, 

 and the line of colhmation of L should pass through the same point. 

 The angles 6 and i/j are measured by the circles E and F. 



§ 9. Experimental details. The shaft A is set horizontal by aid of 

 a level. The colhmator is adjusted optically by an auto-collimating 

 method. The plane of the grating is set horizontal by a level, and 

 the shaft is then turned through 90°, as measured by the circle E, 

 so that the plane of the grating is vertical. Light from a flame is 

 then reflected by a plate of glass held at 45° past the cross-wires 

 and through the lens on to the grating, and the colhmator, pre- 

 viously set for "infinity," is adjusted so that the image of C, the 

 intersection of the wires, coincides with C itself. If the coincidence 

 is recovered when the grating shaft is turned through 180°, the 

 plane of the grating is parallel to the shaft. The line of colhmation 

 is then both horizontal and also perpendicular to the grating shaft. 



The line of collimation of the goniometer is set perpendicular 

 to the goniometer shaft B by an optical method. An auxihary col- 

 limator, set for "infinity," is placed so that it is approximately 

 perpendicular to the shaft. A plate of plane parallel glass is attached 

 to the circle F near its centre so that its faces are approximately 

 parallel to the shaft. It is convenient to make the shaft vertical; 

 the glass plate can then be supported on a small levelhng table 

 resting on the circle. By adjusting both the colhmator and the 



