THE RANGE OF NATURE's OPERATIONS. 213 



of a niilliin or millimeter. Subsection u includes those larger meas- 

 ures which men have also in everyday use — from tenths of a meter up 

 to kllems or kilometers. The third subsection wj, from millims 

 (millimeters) down to tenths of a micron, covers the entire range of 

 the microscope, and indeed travels somewhat beyond the gtasp of 

 that instrument, since the smallest interval at which two objects can 

 be seen as two by the ]>est immersion objectives supplemented by 

 the best immersion condensers, and most carefully handled, is but 

 little less than two-tenths of a micron, which is the one hundred and 

 twent3^-seven thousandth of an inch; whereas subsection Oiv extends 

 twice as far, i. e., down to one-tenth of a micron. This brings us 

 within the border of the next group — the group of molecular inter- 

 vals — almost all of which lie farther beyond the reach of the microscope 

 than microscopic objects lie beyond the grasp of the naked eye. 



GROUP D (molecular QUANTITIES). 



On the borderland between groups C and D we find the lengths of 

 waves of light, all of which can be represented by numbers inserted 

 in the column which is the extreme right-hand column of Group C 

 and the extreme left-hand column of Group D. The wave-lengths of 

 visible light extend from a little less than 4 seventh-metrets to a little 

 less than 8 seventh-metrets. The ultra-violet light which reaches the 

 Earth from the Sun carries us down to about 3 seventh-metrets; the 

 light which has been explored by Professor Hartley extends the range 

 nearly dow^n to li seventh-metrets. and Professor Schumann has got 

 down to light whose wave-length is about 1 seventh-metret. Thus the 

 wave-lengths of light come all of them upon the column which, in our 

 table, is on the border between microscopical magnitudes and molecular. 

 Almost the only true molecular length long enough to be measured in 

 this column is the average free path in attenuated air or in some other 

 gases. On the other hand, when air is as dense as it is at the surface 

 of the Earth, the average lengths of these free paths has to be recorded 

 in the next column (the column of eighth-metrets), and may be consid- 

 ered as about the longest of legitimate molecular intervals. Accord- 

 ing to Maxwell's determinations, it seems to be about 7^ eighth-metrets. 

 The wave-lengths of Rontgen rays perhaps extend into this colunni. 



One or two luiits in the next colunui, the column of ninth-motrets, 

 may Ije taken as about the average interval at which the molecules of 

 ordinary air are spaced; and a unit or two in the following column, 

 that of tenth-metrets, is about the average spacing of the chemical 

 atoms of which solids and liquids consist. It will be seen that none of 

 these intervals extend beyond Dw, the subsection of large molecular 

 magnitudes. 



When we attempt to penetrate further wc find that we can only 

 obtain a glimpse of those more fundamental events in Nature, the size 



