A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 27 



On now carefully working by hand the milled head of the endless screw s, the 

 microscope and its black-glass interference disc g a will move parallel to themselves to 

 an extent invisible to the naked eye, but which is instantly interpreted by the steady 

 movement of the interference bands parallel to themselves, without the slightest twist 

 or alteration of their width ; and reversal of the direction of rotation of the screw 

 causes the instant reversal of the direction of movement of the bands. 



On attaching one of the shafts to the endless screw and to the large control-wheel, 

 the traverse of the microscope and of g 3 can be equally and much more conveniently 

 and delicately effected and controlled, in the manner already fully described in the 

 last section. The calculated number of bands, namely 15 for red hydrogen light, pass 

 most accurately for each revolution of the wheel, and the steadiness and " dead beat " 

 nature of the motion is very pleasing. 



Indeed, the most satisfactory thing about the whole interference method is that the 

 slightest fault, productive of inaccuracy of measurement, is at once visibly indicated 

 by irregular movement, twisting, alteration of width, or complete disappearance of the 

 bands. The fact that during the operation of measuring the difference of length 

 between two bars involving the traverse of the microscope through the length 

 corresponding to this difference the bands move with the precision which has just 

 been stated, is the final proof of the success of the method and of the instrument now 

 described. 



The author desires to record his appreciation of the most kind help and 

 encouragement invariably received from the Deputy Warden of the Standards, 

 Major P. A. MAcMAHON, F.H.S., to whose initiative the whole of this advance in fine 

 measurement at the Standards Department is due. Further, to express his deep 

 indebtedness to Mr. F. STANLEY READ of the Standards Department, and to 

 Mr. J. SKINNER of Messrs. Troughton & Simms, for untiring aid and many excellent 

 suggestions during the design and construction of the apparatus. The author's thanks 

 are in a special manner due to Mr. JAMES SIMMS, the head of this firm, which lias had 

 a long historic connection with the Standards Department, for placing his unique 

 experience and the very highest constructive skill at the service of the Board of Trade 

 in regard to this instrument. Mr. J. H. AGAR BAUGI-I and Dr. DRYSDALE have also 

 afforded invaluable assistance in connection with the unique electric thermostat. 

 Moreover, to the officials of H.M. Office of Works the heartiest thanks are due for 

 very material help in designing the stone foundations, and in the electric installation 

 of the new comparator room, as well as for the admirable manner in which the work 

 has been executed. 



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