30 



DR. A. E. H. TUTTON : 



and in view of the degree of accuracy claimed for the whole result, the fraction of an 

 interference band, it is difficult to see how one could be sure of this when as many as 

 15 whole bands are included in the irregular width of the defining lines, as the author 

 has verified, as regards that on the British iridio-platinum yard, by actually observing 

 these bands to pass while the width has been traversed on several occasions. 



Three years ago the author was attracted by a paper by E. M. NELSON in the 

 ' Journal of the Royal Microscopical Society ' (1906, p. 521) concerning " The Limits of 

 Resolving Power for the Microscope," in which reference was made to some extremely 

 fine rulings, ranging from 10,000 to the inch to 120,000 to the inch the latter being 

 on the extreme verge of possible resolution with the highest-power immersion 

 objectives yet constructed made with an exceedingly fine diamond point on glass by 

 Mr. H. J. GRAYSON of Melbourne, employing a ruling machine of his own construction. 



The author obtained some of these 

 rulings from Messrs. II. and J. Beck, 

 Mr. GRAYSON'S agents, and was greatly 

 surprised and delighted with their truly 

 wonderful sharpness and the clear, un- 

 broken continuity of their edges. Mr. 

 GRAYSON was approached and most 

 kindly made for the Standards Depart- 

 ment a number of experimental rulings 

 on the scales of .40,000, 50,000, and 

 60,000 lines to the inch on glass, silvered 

 glass, speculum metal, gold, silver, 

 platinum-indium, Baily's metal, and 

 invar. Each ruling consisted of five of 

 the lines in question, ruled parallel to 

 each other according to one of the above- 

 mentioned scales, together with a pair of 



Fig. 13. The location signal. 



"finder'' lines parallel with the five rulings, one on each side of them at a suitable 

 interval, and much thicker than these five lines, for the purpose of readily finding the 

 lines under a preliminary low power ; and also a pair of lines transverse to the five 

 parallel rulings and crossing them rectangularly for the purpose of identifying a 

 central portion of the rulings. 



The appearance of the essential central part of such a series of rulings under the 

 iVinch dry lens, provided with each of the two microscopes described in the 

 preceding communication, is shown in fig. 13. The author's idea was to constitute 

 the central third line, of the five lines composing such a ruling, the defining line. A 

 ruling such as is represented in fig. 13 the author proposes to refer to as a " location 

 signal " to distinguish it from the single defining line hitherto employed. 



It may at once be stated that the author found the rulings on speculum metal, 



