354 Lord Rayleigh on the Accuracy of Focus 



Table VI. 



Date. 



Standard of 

 Comparison. 



B.A.U. in M. 



M. in B.A.U. 



April 9 



Flat. 

 !f! 54. 



Flat. 



Flat. 



t 54. 



$54 and 20. 



Flat and 20. 



Flat and 20. 



1-04814 

 104806 

 104791 

 104820 

 1-04811 

 1-04801 

 104824 

 1-04797 



•95407 

 •95413 

 •95427 

 •95402 

 •95410 

 •95419 

 •95399 

 •95422 



„ 15 





16 





„ 17 







Mean . . . . , 



1-04808 



•95412 





These numbers, it will be seen, agree with those found by 

 Lord Rayleigh and Lorenz, and differ greatly from Strecker's 

 values. 



They represent the results of the comparison of a German- 

 silver "copy of Strecker's tubes with the B.A. units, including 

 the actual coil employed by Strecker, ^ No. 54 ; while 

 Strecker's numbers give the result of the comparison of his 

 tubes directly with No. 54. 



I have not been able to explain the discrepancy. Herr 

 Strecker suggests in a letter to me on the subject, that the 

 resistance of his coil No. 13 has changed somewhat in the 

 year which has elapsed since his comparisons were made. At 

 present I am in correspondence with Prof. F. Kohlrausch, of 

 Wiirzburg, in whose laboratory Strecker's experiments were 

 conducted, and hope to obtain another copy of Strecker's tubes, 

 or at any rate to have another comparison made between his 

 tubes and the B.A. standards. 



XL. On the Accuracy of Focus necessary for Sensibly Perfect 

 Definition, By Lord Rayleigh, F.B.S* 



IN my " Investigations in Optics " f I have examined the 

 effect upon definition of small disturbances of the wave- 

 surfaces from their proper forms. It follows, for instance, 



* Communicated by the Author, 

 t Phil. Mag. 1879 & 1880. 



