422 Dr. G. J. Stoney on the 



The sixthet-metre is identical with tbe micron spoken of 

 above. Many writers represent it by the symbol /jl. 



The ninthet-metre is the thousandth part of a micron. It 

 has sometimes been called the micro-millimetre, and is by 

 some writers represented by the symbol /u/j,. 



The tenthet-metre is the same measure as is usually called 

 the tenth-metre. It has also sometimes been called the 

 tenth-metret. 



IX. Of the Smallest Magnitudes that have been Measured. 



The most minute magnitudes that have been actually 

 measured are differences of wave-length. These can be de- 

 termined with truly astonishing precision by observations with 

 the diffraction-grating spectroscope, so much so that they 

 carry us down to magnitudes that are fractions of the diameter 

 of a gaseous molecule. The observation is most easily made 

 in the case of close double lines. En these the interval be- 

 tween the two constituents is due to the difference of their 

 wave-lengths, and by measuring the former the latter can be 

 ascertained. Thus, an interval of one degree on Angstrom's 

 or Rowland's map indicates a difference of wave-length 

 amounting to a tenthet-metre, which, as we know, is the 

 ordinate of our standard gauge at a distance of a millimetre 

 from its end. But lines have been seen to be double with 

 Prof. Rowland's gratings, in which the separation of the two 

 constituents is not more than from l/30th to l/100th part of 

 a degree. In the latter case the difference of wave-length is 

 only one twelfthet-metre. This is the ordinate of our gauge 

 at a distance from its apex which is little more than the 

 diameter of a single blood-corpuscle, and may be taken to be 

 the smallest measurement that can as yet be directly effected 

 with certainty. 



The following is a list of close double lines which I have 

 myself seen in the solar spectrum with a small Rowland's 

 grating*:— 



The solar line b 3 , and a multitude of other close doubles, 

 shown as such on Rowland's map (2nd Series, 1888). 



The less refrangible of the tw r o E lines. 



The line in the E group at \= 5264*4 of Rowland's 

 scale. 



* The grating is flat, nearly an inch and three-quarters long, contains 

 about 25,000 lines, and the observations were made in the fifth spectrum. 

 In this spectrum the image is formed by bringing together every fifth 

 wave of light out of a series of 125,000 consecutive waves, and theruling 

 must be sufficiently accurate to effect this. 



