214 THE RANGE OF NATUKE's OPERATIONS. 



of which or the niiig-e of which ha< to l>e measured in the next three 

 cohimns, i. e., iu tenthets of the decimeter, of the centimeter, of the 

 millimeter. These all come into subsection ?•. the subsection of meduim 

 molecular mtiguitudes. That there are events of this kind o-oinof on 

 unremittiusjly within every chemical atom is indicated to us by the 

 lines in the spectra of the chemical elements, for these are caused In- 

 such events. Here, at present, human knowledo^e stops. The whole of 

 the work which Nature is carryingr 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 mathemati- 

 cians may doubtfully attempt to probe it: and in all such conjectures the 

 speculator has to substitute something- very nnich simpler for what is 

 really going- on. However. Group D is represented in our diagram as 

 including another subsection, w. going 10,(HX> times farther still, in 

 order by this extension to provide for the possibility of future dis- 

 coveries which we hope may some day be realized. 



Very little is known about the events going on within chemical 

 atoms, of which we have found that the i"ange is to be measuivd in 

 tenthet-decimeters. tenthet-centimetei^s. or teuthet-milliuieters, 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 iis differences of wave- 

 length. This is truly astonishing when we remember that we are here 

 measuring lengths that are from 1(X».(K» to l.(K>>.iHK> 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 spec- 

 trum 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 interferometer. By the spectrometer measures 

 can be carried at all events as far as the fiftieth of a tenthet-meter. i. e.. 

 as far as to one or two tenthet-ceutimeters. while with the interferometer 

 determinations can probably l>e carried one step of our scale farther. 

 i. e.. to one or two tenthet-millimeters. Here, for the present, our 

 powers end; and we can not fail to be impressed by the extraordinary 

 accm-acy which has been attained in measuring wave-lengths by the 

 methods spoken of above. It is a degree of accuracy which ascer- 

 fciins the length of a wave of light within a millionth of its entire 

 length, thus equaling and even surpassing the best results o]>tained 

 when comparing with excessive care international standard yards or 

 meters; in which a determination within one fifthet (the ICKl.OOOth) of 

 the whole length is probably the most that can be f allv relied on. 



