32 DR. A. E. H. TUTTON: 



would have been considered impossible to see lines only a wave-length apart. On 

 traversing the microscope which carried the black-glass interference plate, from the 

 position in which the first ruling of one of these location signals was adjusted between 

 the parallel pair of spider-lines to that in which the fifth ruling was similarly 

 adjusted the microscope having thus travelled over four wave-length spaces exactly 

 eight interference bands (each corresponding to half a wave-length, JJ^JQ inch) in 

 red hydrogen light passed over the reference spot ; this was true however many 

 times, and in whichever direction, the traverse was performed, and was precisely the 

 number expected within a very small fraction, if the rulings were truly spaced -foooo 

 of an inch apart. 



Thus the interval between two lines ruled 40^0^ mcn apart corresponds to the 

 passage of two interference bands in red hydrogen or cadmium light, past the reference 

 spot in the centre of the field of view of the interferometer telescope, a very con- 

 venient fact. The lines themselves are almost as sharp as the spider-lines of the eye- 

 piece micrometer, and there is no trace of roughness of edge ; they are also narrower 

 than the white interspaces between the lines, so that the width of a line is less than 

 half a wave-length. 



With the -f^ -inch dry objectives, specially constructed under the direction of 

 Mr. Conrad Beck, the resolution is excellent, and the working distance so relatively 

 great for so high a power as to leave a clearly visible air film between the cover glass 

 and the objective, quite a safe working distance, in fact, for all traversing and 

 adjusting purposes. With an oil immersion objective of the same power the definition 

 is simply surprising. 



The lines at 60,000 to the inch are not so satisfactorily resolvable without an 

 immersion lens, which would be inconvenient for the purpose in view ; and the rulings 

 50,000 to the inch are still a little subject to the same criticism. But the 40,000 

 rulings to the inch are perfect for the purpose, their apparent separation being so 

 considerable that one of the spider-lines of the micrometer eye-piece can readily be 

 placed between two adjacent ruled lines and still leave a clear white space on each side 

 of the spider-line. The sharpness of the edges of any particular line thus ruled on 

 speculum metal is quite comparable with that of the edges of the spider-line itself. 

 All these considerations, combined with the fact that 40,000-to-the-inch rulings are 

 approximately single wave-length rulings (for red light), render the location signals on 

 the 40,000-to-the-inch scale remarkably suitable for defining marks on standard bars, 

 the middle line of the five being the defining line. This is equally true whether they 

 are employed in the usual manner merely as end marks, or as interval marks in a 

 scheme of stepping-off by repeated doublings, for the determination of the total 

 number of wave-lengths in the yard or metre. Such a scheme would have the great 

 advantage that the final end-defining marks would be of the same accurate character 

 as all the intermediate ones. 



The author has elaborated such a scheme for the determination of the number of 



