.] UNITS AND STANDAKDS OF MEASUREMENT. 319 



Natural System of Standards. 



Quite recently Professor Clerk Maxwell has suggested 

 that the vibrations of light and the atoms of matter might 

 conceivably be employed as the ultimate standards of 

 length, time, and mass. We should thus arrive at a 

 natural system of standards, which, though possessing no 

 present practical importance, has considerable theoretical 

 interest. " In the present state of science," he says, " the 

 most universal standard of length which we could assume 

 would be the wave-length in vacuum of a particular kind 

 of light, emitted by some widely diffused substance such 

 as sodium, which has well-defined lines in its spectrum. 

 Such a standard would be independent of any changes in 

 the dimensions of the earth, and should be adopted by 

 those who expect their writings to be more permanent than 

 that body." a In the same way we should get a universal 

 standard unit of time, independent of all questions about 

 the motion of material bodies, by taking as the unit the 

 periodic time of vibration of that particular kind of light 

 whose wave-length is the unit of length. It would follow 

 that with these units of length and time the unit of 

 velocity would coincide witli the velocity of light in empty 

 space. As regards the unit of mass, Professor Maxwell, 

 humorously as I should think, remarks that if we expect 

 soon to be able to determine the mass of a single molecule 

 of some standard substance, we may wait for this deter- 

 mination before fixing a universal standard of mass. 



In a theoretical point of view there can be no reasonable 

 doubt that vibrations of light are, as far as we can tell, the 

 most fixed in magnitude of all phenomena. There is as 

 usual no certainty in the matter, for the properties of the 

 basis of light may vary to some extent in different parts of 

 space. But no differences could ever be established in the 

 velocity of light in different parts of the solar system, and 

 the spectra of the stars show that the times of vibration 

 there do not differ perceptibly from those in this part of 

 the universe. Thus all presumption is in favour of the 

 absolute constancy of the vibrations of light absolute, 

 that is, so far as regards any means of investigation we are 



* Treatise on Electricity and Magnetism, vol. i. p. 3. 



