Dr Searle, Experiments with a plane diffraction grating 105 



To identify C, a pin is mounted on a carriage so that its tip 

 coincides with E. The carriage is moved along the bench so that 

 the tip touches the scale AB. The point of contact is C. If L is 

 adjusted on its carriage so that / coincides with C for both cases, 

 then M, N lie on EC. 



§ 8. Other experimental details. The scale ^ B is set perpendicular 

 to the bench. A set square XYZ, with the right angle at X, is held 

 with XY in contact with AB. A pin is held close to XZ. If, when 

 the carriage D is moved along the bench, the distance from the 

 pin to XZ is constant, AB is correctly placed on its carriage. The 

 scale must be horizontal and the slit vertical. The plane of the 

 grating can be set perpendicular to EC optically. The lens L is 

 removed and a small triangle of white paper is fixed to AB so that 

 a vertex coincides with C. The grating G is placed midway be- 

 tween C and E and is adjusted on its carriage so that the image 

 of C by reflexion at G coincides with E. To allow a close test of 

 parallax, a few grains of lycopodium may be placed on AB when 

 the image of the slit does not fall on a dividing line. 



§ 9. Practical example. Using a grating with d = 1-7526 x 

 10~*cm., the following results were obtained: 



The image of first order was used; thus i = \. 



Bench reading 118-50 cm., glass scale readings 97-52, 73-84 cm. 

 141-21 cm., „ „ „ 89-64, 82-18 cm. 



Hence 



ih = \ (97-52 - 73-84) = 11-84 cm., y^ = | (89-64 - 82-18) = 3-73 cm. 

 Also x^- x^^ 141-21 - 118-50 = 22-71 cm. 



Hence tan cf) = {y^ - y2)l{x^ - x^) = 8-11/22-71 - tan 19' 39' 7". 

 Then X = dsin (p/i = 5-894 x 10-^ cm. 



§ 10. Experiment with an astigmatic incident beam. The experi- 

 mental test of the results of § 4 is a good exercise in optical manipu- 



lation. Fig. 5 is a plan of the apparatus. Two cross- wires, inter- 

 secting in E, are fitted into a tube turning about a horizontal axis 



