Stonry— The known part of Nature's Work. 87 
the size of which or the range of which has to be measured in the 
next three columns, 7. e. in tenthets of the decimetre, of the 
centimetre, of the millimetre. These all come into subsection 2, 
the sub-section of medium molecular magnitudes. That there are 
events of this kind going on unremittingly within every chemical 
atom is indicated to us by the lines in the spectra of the chemical 
elements ; for these are caused by such events. Here, at present, 
human knowledge stops: the whole of the work which Nature is 
carrying on at still closer quarters, although we are well aware 
that it must lie at the basis of all the rest, is totally hidden from 
our view, except so far as the speculations of mathematicians may 
doubtfully attempt to probe it; and in all such conjectures the 
speculator has to substitute something very much simpler for what 
is really going on. However, Group D is represented in our 
diagram as including another subsection, wv, going 10,000 times 
farther still; in order by this extension to provide for the 
possibility of future discoveries which we hope may some day 
be realised. 
Very little is known about the events going on within chemical 
atoms, of which we have found that the range is to be measured in 
tenthet-decimetres, tenthet-centimetres, or tenthet-millimetres, and 
even the fact that there are such events lies near the limit of our 
knowledge; and yet these excessively minute quantities can be 
dealt with accurately when they present themselves as differences 
of wave-length. This is truly astonishing, when we remember 
that we are here measuring lengths that are from 100,000 to 
1,000,000 times smaller than the most minute interval that can be 
detected by the microscope—as much smaller than a micron, as a 
tenth or hundredth of an inch is less than three quarters of a mile. 
Nevertheless these lengths can be determined with precision because 
the position of a line in the spectrum depends on its wave-length, 
and the difference of the wave-lengths of the closest lines which can 
be photographed as double is excessively small; and again, because 
two rays with a still smaller difference of wave-length may give 
rise to interference effects which can be detected by the interfero- 
meter. By the spectrometer measures can be carried at all events 
as far as the 50th of a tenthet-metre, ¢.¢. as far as to one or two 
tenthet-centimetres, while with the interferometer determinations 
can probably be carried one step of our scale farther, 7.¢. to one or 
