Dr Seaiie, Experiments on focal lines formed by a zone plate 343 



Since JPp tan- (9 = 1-4886, when j9= +1 and (9=45°, and since 5i -Ci = -9U0, 

 Fq" and y^^^ are positive, and thus ^^ has the same sign as ^i- Using the 

 real focal lines corresponding to p = l, we verified that ^o/'^i is positive. 



§ 10. Fourth practical example. In these experiments a true 

 image of the object at K (Fig. 5) was produced; the direction of 

 the axis of the zone plate was so adjusted that the astigmatism 

 due to its obliquity balanced the astigmatism due to the lens. 



The mean value found for F^ in § 7 is 1-5133 dioptre. Hence Fj^ (1 +sec2 6) 

 is at least equal to 3-0266Z) and i^,(l+sec-^) and i^g ( 1 + sec- (9) are still 

 larger. Since B^ -C\ =-9110, X and p are positive or negative together. 



In the first set of observations, •v/'i=0. The slit was made horizontal and, 

 with ^ = 0, the telescope was f ocussed on an image for which p= -1, - 2 or 

 - 3; each image was virtual. This image is in the same place as the image which 

 is formed by horizontal focal lines when is changed from zero. The slit is 

 then made approximately vertical and the zone plate is turned until 6 is such 

 that the image is again in focus. The image is then in focus for all directions 

 of the sUt. 



In the second set of observations, \//'i =|7r, and the images for which p = \, 

 2 or 3 were used; these images were real. 



The calculated values of 6 were found from (30), using J^i = 1-5133 and 

 5i-Ci = -91 10 dioptre. 



Corrections in Papers by Dr G. F. C. Searle. 



(i) "A bifilar method of measuring the rigidity of wires" {Proc. Camb. 

 Phil. Soc. Vol. XX. p. 61). Equation (10) on p. 67 should read 



When this correction is made, the quantity C remains unchanged, but the 

 values given for P and Q in (18) and (19) must be multiplied by 1 +C. In the 

 practical example (p. 69) P is changed from - -0028 to - -0029 and Q from 

 - -0045 to - -0046. There will be corresponding small changes in the last 

 column of the table of results on p. 68. 



(ii) "Experiments with a plane diffraction grating"' (Proc. Camb. Phil. 

 Soc. Vol. XX. p. 88). An error afl'ecting §§ 10, 12 of this paper is noticed in 

 a footnote to § 6 of '"Experiments on focal lines formed by a zone plate" 

 {Proc. Camb. Phil. Soc. Vol. xx. p. 340). 



