334 



EECEEATIYE SCIENCE. 



and even from the same source, being marked 

 with twice or thrice the degree of fineness of 

 others, to remedy which it was necessary that 

 some absolute standard should be obtained. 

 The application of the principle of machine- 

 ruling led to its solution in the subject of the 

 present paper. 



Slips of glass ruled with lines, 100 and 

 1000 to the inch, are in common use, and are 

 invaluable in ascertaining the degree to which 

 any object is magnified ; without them we 

 should be unable to estimate the closeness of 

 the rxiling in M. Nobert's wonderful produc- 

 tions. M. Froment, of Paris, succeeded in 

 ruling lines on glass 25,000 to the inch ; in 

 this path he was followed by M. F. A. 

 Nobert, of Barth in Pomerania, M'ho sent to 

 the Great Exhibition, in 1851, some slips ruled 

 ■n ith bands of lines of different degrees of 

 fineness ; in the first band the lines were 

 11,000 to an inch, in the last 50,000 in the 

 same space. The bands in these his first- 

 exhibited productions, were ten in number, 

 and having been separately estimated by 

 several observers of high repute, these num- 

 bers are generally accepted as correct. 



M. Nobert's later productions surpass 

 even these ; as at present supplied, they con- 

 sist of a slip of glass (Fig. 1), on which, by 



Fig. 1. 



careful observation with the naked eye, may 

 be observed what looks like a single shadowy 

 line, sparkling in bright light with the play of 

 prismatic colours. By moderate power (50 

 diameters), with good glass and careful illu- 

 mination, twenty distinct bands may be 

 counted, the finer, however, very shadowy 

 and evanescent. With 100 diameters, the 

 four coarsest shoiild be shown in distinct 



lines (Fig. 2) ; as the power is increased, more 

 and more of the bands will be resolved, till 



with 500 diameters the separate lines com- 

 posing all but one or two of the finest of the 

 bands should be clearly seen: for satisfactory 

 resolution of the latter a power of 800 dia- 

 meters, with most skilful management of the 

 accessories, is required. Fig. 3 may serve to 

 give some idea of their appearance then, 



iniin 



Fig. 3. 



though it is impossible to imitate closely the 

 exquisite delicacy of the lines. 



The difficulty of viewing, and still greater 

 of counting, lines so close, is such that ob- 

 servers are not at present equally agreed in 

 their measurements of the later series. As 

 furnished by a friend, in whose accuracy we 

 place great reliance, from his extensive prac- 

 tice in such investigations, the range is from 

 13,000 in band No. 1 to 80,000 in band No. 20. 



The extreme delicacy and regularity of 

 these lines, with their closeness, are calcu- 

 lated to excite admiration in the highest de- 

 gree. We run over the numbers glibly, as 

 we do the " millions of miles" of tlie astro- 

 nomer, but to form any conception of lines, 



