ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 239 



those on the platinuni-iridium copy of the imperial standard yard. Yet 

 even each of these has a thickness equivalent to fifteen interference 

 bands. The defining lines on the imperial yard itself are three times as 

 coarse. Hence we have now arrived at that stage in the competition 

 between defining lines and refinement of measurement when the latter 

 has far surpassed the former. It was for this reason that the author 

 took up the investigation of wave-length rulings, with the idea of their 

 possible use as defining lines commensurable with the increased refine- 

 ment of measurement. Mr. H. J. Grayson, of Melbourne, whose 

 wonderfully fine rulings have recently been much discussed in micro- 

 scopic circles, has kindly made a number of rulings of j^oif m - fi ne " 

 ness, which preliminary experiments indicated as feasible for the 

 required purpose, on polished speculum-metal and platinum-indium, 

 which appear, particularly the former, perfectly satisfactory. The 

 Ttfixoo m - being the wave-length of red hydrogen or cadmium light, the 

 distance between two lines ruled at this interval corresponds to only 

 two interference bands. "With the ^ in. dry objectives, the lines, more- 

 over, are as cleauly cut as spider-lines, and the thickness of a line is 

 less than half a wave-length. Five such lines are ruled in succession, 

 the central one being considered as the defining line. A strong finder- 

 line is ruled on each side of the five, and two other strong ones at right 

 angles in order to localise a central part of such a system. It appears 

 perfectly feasible to carry out a stepping-off process for the counting of 

 the total number of wave-lengths of cadmium red light in the British 

 yard, in which such rulings would take the place of the glass plates of 

 the Michelson or Fabry and Perot etalons, a base line of the thirty- 

 second part of an inch being first actually counted in bands with the 

 aid of the interferometer, between limits defined by two such systems of 

 rulings. The final fraction of every stage in such a process could be 

 absolutely checked by the interferometer in all cases where Michelson 

 found it possible to do so, that is, so far as interference bands are 

 still visible, about 4 in. ; and, as it has already been proved that the 

 accuracy with the rulings is almost as great as with interference bands, 

 this checking ceases to be as imperative as when only the coarse existing 

 defining lines are available. Hence, the future before these rulings 

 appears likely to be both interesting and important. 



On the Production of Micrometric and Diffraction Rulings.* 

 Henry J. Grayson says : Some years ago I had occasion to use some 

 finely-ruled glass plates, not exceeding 0" 01 in. thickness, the lines upon 

 them ranging from ' 02 in. to ■ 004 in. apart. These, I found, were not 

 readily obtainable commercially, so that I had to devise some method of 

 producing them for myself. After a few experiments, I soon found I 

 had no difficulty in ruling lines greatly exceeding in fineness and 

 accuracy any of the kind I had hitherto seen, and, as the matter was 

 interesting to me from a microscopical standpoint, I pursued it apart 

 from my immediate requirements. 



The apparatus I first devised and used was exceedingly simple in 

 principle, and consisted essentially of a fine steel screw and wedge of 

 glass, the incline of the latter bearing some definite ratio to the pitch of 



* The Microscope, i. (1909) pp. 4-11. 



