THE GRAYSON RULINGS 

 By Dr. A. E. H. Tutton, F.R.S. 



It must have been with the deepest regret that workers with the 

 microscope heard of the premature demise of Prof. II. J. Grayson, 

 of Melbourne, the remarkably gifted maker of the well-known 

 " Grayson Rulings." Those who have used the rulings have been 

 struck with both their accuracy as regards spacing, and the extra- 

 ordinary sharpness of each individual line, especially in the case of 

 those on speculum metal. The truly wonderful guiding of the 

 diamond point by the late Prof. Grayson's own unique master hand, 

 no less than the perfect construction of his ruling machine, which 

 enabled such accurate spacing to be obtained, have never ceased to 

 impress those who have worked with these rulings. Their merit 

 begins at the point where the other rulings so well known to us, such 

 as those of Rowland and of Michelson, leave the field, namely, above 

 20,000 to the inch. His extreme rulings of 120,000 to the inch, 

 are a direct challenge to the microscope, for they represent its 

 highest resolving power. While these wonderful rulings, and those 

 only a degree less impressive of 100,000 and 80,000 to the inch, are 

 of great use to us in studying high resolution, with natural micro- 

 scopic objects presenting detail of great minuteness, and also in 

 actual calibration and measurement of the detail of objects of sucH 

 extreme minuteness, it is probably with the more moderately spaced 

 rulings of 60,000 and 40,000 to the inch that the most important 

 work is to be done. 



The writer has already called attention, in his memoir* to the 

 Royal Society on the Interference Comparator for Standards of 

 Length, to the fact that the Grayson rulings of 40,000 to the inch 

 spacing are capable of becoming of great importance in metrology, 

 as fiducial marks, the middle one of five such rulings forming an 

 excellent signal-mark. For, as was pointed out in the memoir, the 

 40,000th of an inch is the wave-length of red light, very close indeed 

 to the exact wave-lengths of the red hydrogen (.^Jyo iiich) or the 

 red cadmium (..yj^y inch) line. Thus, the space between any 

 two successive lines of the 40,000 to the inch rulings corresponds 

 practically exactly to the passage of two interference bands (two 

 complete interference-band spacings) in red hydrogen or cadmiuni 

 light. That this is true of the late Prof. Grayson's rulir^ 

 labelled by him as 40,000 to the inch, has been proved by the 

 writer by direct measurement against the interference bands, on 

 the Comparator at the Standards Department. These more 

 moderately finely spaced rulings are admirably resolved by the 

 l/15th inch dry objective supplied for the purpose by Mr. Conrad 

 Beck. The lines, indeed, as seen through the fine-movement micro- 



* Phil. Trans., A., 1910, 210, 30. 



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