THE EXTREME INFRA-RED RADIATIONS. 165 



can be made by either of the two following methods : The wave lengths 

 may be plotted as abscissre upon a simple arithmetical scale, or for 

 them may be substituted their logarithms, so as to make a geometrical 

 scale in which each octave of light occupies an equal space in counter- 

 part to the keyboard of a piano. The second method is the more 

 rational, inasmuch as it gives to each part of the spectrum a space more 

 in proportion to its importance. 



The diagram (tig. 2) has been prepared in conformity with the second 

 scheme. 1 



The light spaces in the diagram correspond to the spectrum now 

 known, while the shaded portions represent spectral regions not yet 

 explored. The light portion grows continually, and there now remains 

 unknown only a small region to remind us of the part formerly in 

 obscurity. 



'The octaves in the figure are numbered arbitrarily aud arranged in the opposite 

 direction to the musical octaves on a piano keyboard. That is, the octaves of radia- 

 tions of more rapid vibration, which may be compared to higher musical pitches, are 

 upon the left. It would be useful to adopt some fixed numbering for the octaves of 

 radiation. 



