34 DR. A. E. H. TUTTON: A WAVE-LENGTH COMPARATOR, ETC. 



The second difficulty is only one of degree, and of the perfection of attainment of 

 the constant temperature of 62 F. with the electric thermostat ; that is, the fact that 

 the rulings now being employed are on speculum metal. When further experience of 

 these rulings has been gained, however, the author feels convinced that we shall be 

 able to obtain them almost equally satisfactorily on highly polished platinum-iridium, 

 and, therefore, assuming the first difficulty overcome, directly on the bar itself. 

 Gold and invar are not so promising, the former being apparently too soft and the 

 latter too hard. 



In conclusion, it may be stated that the author's location signal, of five Grayson 

 rulings on the 40,000-to-the-inch scale, as above described, and as shown in fig. 13 

 under the -j^-inch power, makes a splendid defining line under the |-inch power 

 employed in observing the Benoit or other coarser ordinary defining marks on standard 

 line-bars. This power is insufficient to resolve the five lines, so that they appear as a 

 single defining line, but of truly wonderful sharpness of edge, as may readily be 

 imagined when their sharpness under the resolving power of the ^5 inch objective is 

 remembered. Even if the high hopes at present entertained as to the use of these 

 rulings as stages in stepping-off the total number of wave-lengths in the yard be not 

 fulfilled, the investigation of the rulings will have fully justified itself in giving us a 

 defining mark of the high accuracy and refinement demanded by the wave-length 

 interference method of measurement. 



The author desires to record his sincere thanks to Mr. GBAYSON for the admirable 

 manner in which he has prepared the special rulings described in this communication, 

 and for the enthusiasm with which he has entered into the author's plans. Also to 

 Mr. CONRAD BECK for his aid in satisfactorily overcoming the problem of their 

 maximum resolution, illumination, and definition. 



