A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 33 



wave-lengths of red cadmium light in the British Yard and proposes to check the 

 results obtained with it by comparison with the Fabry and Perot method, to which 

 the apparatus described in the preceding communication lends itself admirably, and 

 which has recently been successfully employed at Sevres to check the results of 

 MICHKLSON and BENOix for the French metre. The total number of wave-lengths in 

 the -j^-inch interval of one of the longer rulings above referred to, corresponding to 

 the transit of about 4200 interference bands, can quite easily be counted as a funda- 

 mental base line, and, of course, the g^-inch interval of 2100 bands more readily still, 

 by direct traverse of the microscope of the wave-length comparator described in the 

 preceding communication, from one location signal defining the interval to the other. 

 By the author's scheme of repeated doubling, a full account of which will be published 

 later when the results are described, and which involves the use of two of the larger 

 1^-inch rulings and nine of the single location-signal rulings, five of the latter 

 mounted on a yard bar and the rest along with the two larger plates on an inter- 

 mediate 4|-inch bar, the whole yard can be eventually stepped off. Each stage thus 

 replaces one of the stages of MICHELSON'S etalons and each is susceptible of absolute 

 checking of the final fraction of a band by direct observation of the interference 

 bands, if this should be found necessary ; as far as preliminary measurements go, the 

 necessity has not proved an imperative one, for practically as great accuracy is afforded 

 with the rulings as with the bands as regards the determination of a fraction of the 

 interval between any two lines or bands. 



Sufficient will have been said to indicate the great possibilities before the use of 

 these Grayson wave-length rulings, and the author desires to be allowed to reserve 

 for a time in his hands their fuller investigation for the purposes of metrology with 

 the aid of Mr. GRAYSON, who, as the man behind the machine, is at present the only 

 person in the world able to produce such magnificent and accurate rulings, delicacy of 

 hand being quite as important and essential a part in their construction as fineness of 

 the diamond point and precision in the ruling machine. 



At present there are two difficulties confronting us, which it is hoped may soon 

 disappear. One is that Mr. GRAYSON is not able to rule a greater length of bar than 

 the 1^-inch strips described in this communication, and, therefore, they cannot yet be 

 ruled directly on the standard bars themselves. This is only a matter of the modifi- 

 cation of the ruling machine, however ; but, as Mr. GRAYSON is in Australia and 

 communication is slow, it has not yet been arranged for, but lies with the immediate 

 future. A very satisfactory method of mounting has been arrived at, however, 

 comparable with the mounting of his glass plates by MICHELSON, so that any criticism 

 on this head is equally applicable to the e"talons. The covering by cemented cover 

 glasses, at first looked at by the author somewhat suspiciously, proves on closer 

 acquaintance with high-power microscope work to present no difficulty whatever, or 

 to introduce any error in actual practice, as " critical " illumination is always achieved 

 with great perfection in the instrument described in the last communication. 



VOL. OCX. A. F 



