342 Dr Searle, Experiments on focal lines forrned by a zone plate 



X= - 2-7046 Z). 



§ 9. Third practical example. In this experiment the directions 

 of the focal lines were observed. 



The apparatus was the same as in § 8, and 6 was 45°. For each value 

 of ^/^l, the direction of the sUt was adjusted so that the image formed by focal 

 lines was as sharp as possible, and the cross-wire of the telescope was set 

 parallel to the edges of the image. The setting of the cross-wire thus depends 

 upon the accuracy of adjustment of the sUt, for a change of direction of the 

 slit changes the direction of the "image," in addition to changing the sharpness 

 of its edges. The zone plate carriage was then moved aside on its slide R 

 (Fig. 5) and the telescope was moved back, until a fine wire stretched across 

 the face of L was in focus. The reading of the circle F, when this wire was 

 vertical, was known. The tube M was tiurned so that this wire was parallel to 

 the cross-wire of the telescope; the direction of the cross- wii-e was then given 

 by the reading of F. In each case the focal Une corresponding to ^ = - 1 was 

 used; this focal line was virtual. 



Two sets of measurements were made. In the first set, when ^^=^, the 

 sUt was vertical. Then ^^ was changed by steps of 15° to 90°, and the image, 

 which corresponds to 5„, was followed up. The inclination of this image to the 

 vertical is ^^^. In the second set, when ^^ =0, the slit was horizontal, and this 

 image corresponds to G^. The inclination of this image to the horizontal is 

 also i/^2- The table below gives the two values found for i/^, and their mean. 

 In calculating ^^ from (29), we take B^ - G^ =0-9110, as found in § 8, and use 

 Y as given in the table in § 8. Since Y is negative when p = - 1, and since 

 B^ - Gi is positive, it follows that \/^2 is negative when yj^^ is positive. 



^i 



