338 Dr Searle, Experiments on focal lines formed by a zone plate 



H upon a carriage sliding on a graduated track R at right angles 

 to the optical bench S. The zone plate can then be moved aside 

 and be replaced at will. The focal lines are observed by aid of a 

 low-power eye-piece or of a telescope T mounted on a carriage 

 shding along the optical bench S. The length of the telescope, 

 which must remain unchanged during any set of observations^ 

 should be adjusted so that an object about one metre from the 

 objective may be seen in focus. A cross- wire which has been 

 properly focussed will enable the observer to secure more definite 

 settings of the telescope or eye-piece. If a telescope is used, the 

 positions of virtual as well as of real images and focal lines can be 

 observed. 



In testing the results of § 3, a small circular hole P in a sheet 

 of metal, illuminated by a sodium flame Z, may form the object. 

 The hole P lies on the line OX passing through and parallel to 

 the length of the bench. The distance PO may be about 2/^. 



;"^N |l Nl I I II I iTi II liil I h II llil III 



Fig. 5. 



A strmg, stretched parallel to the length of the bench, may be 

 used m settmg P on to the line OX and in making the axis of the 

 telescope comcide with OX. The zero reading of the circle H is 

 found by holding a set square against the zone plate and adjustino- 

 the table so that the appropriate edge of the square is parallel tS 

 OX; suitable optical apphances would give greater accuracy of 

 adjustment. The angle NOX is d. When e = 0, the images of P 

 will be found on moving the telescope along the bench By com- 

 paring the bench reading of the telescope for any image with its 

 reading when focussed on the zone plate, the distance of the image 

 from IS found. The distance is called positive when the image is 

 real. 



If ^ is now increased from zero, the circular image of P will be 

 drawn out into a horizontal /ocaZ line at a point R on OX and the 

 position of R will coincide with that of the image of P for 6 = 

 There will also be a vertical focal line at Q on OX, but this will 

 move towards as 6 is increased, the "power" of the zone plate 

 changing from F^ to F^ sec^ 6 for this Hne. If PO = u, and if for 

 any value of 6, and for any value of p, the distances QO RO are 



