Stoney — Appreciation of Ultra-Visible Quantities. 537 



prevents tlie opposite sides of the ring from being seen as two 

 objects. They accordingly look like disks.^ 



VIII. — Of the Nomenclature or Small Measures. 



It will often be found convenient to connect the proposed stan- 

 dard gauge with another useful way of describing small magni- 

 tudes. Let us understand by a sixthet a unit in the sixth place 

 of decimals, i. e. the fraction 1/10", and let us use the phrase 

 sixthet-metre, or metre-sixthet, to mean the sixthet of a metre, in 

 the same sense in which we say half-inch or quarter- inch to mean 

 the half or quarter of an inch. 



We can then conveniently express the following table of 

 equivalents. 



The ordinate of the standard gauge, at a distance 



Of ten metres from the apex, = a sixthet-metre. 



Of one metre, = a seventhet-metre. 



Of one decimeter, . . . . = an eighthet-metre. 



"Of one centimetre, . . . . = a ninthet-metre. 



Of one millimetre, . . . . = a tenthet-metre. 



The sixthet-metre is identical with the micron spoken of above. 

 Many writers represent it by the symbol ju. 



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 mx. 



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

 tenth-metre. It has also sometimes been called the teuth-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 determined with 

 truly astonishing precision by observations with the diffraction 

 grating spectroscope; so much so that they carry us down to mag- 

 nitudes that are fractions of the diameter of a gaseous molecule. 

 The observation is most easily made in the case of close double 

 lines. In these the interval between the two constituents is due 



^ The white pearl-like specks which take the place of the dots when they are a little 

 out of focus, must not be mistaken for their beiag seen as rings. They are an optical 

 effect, and of larger size. 



