NATURE 705 



scalar quantities. The elementary phenomenon is then the action of 

 an isolated body. Or suppose, again, it is a question of small move- 

 ments, or more generally of small variations which obey the well- 

 known law of mutual or relative independence. The movement ob- 

 served will then be decomposed into simple movements for example, 

 sound into its harmonics, and white light into its monochromatic 

 components. When we have discovered in which direction to seek for 

 the elementary phenomenon, by what means may we reach it ? First, it 

 will often happen that in order to predict it, or rather in order to 

 predict what is useful to us, it will not be necessary to know its mech- 

 anism. The law of great numbers will suffice. Take for example the 

 propagation of heat. Each molecule radiates towards its neighbor 

 we need not inquire according to what law; and if we make any 

 supposition in this respect, it will be an indifferent hypothesis, and 

 therefore useless and unverifiable. In fact, by the action of averages 

 and thanks to the symmetry of the medium, all differences are levelled, 

 and, whatever the hypothesis may be, the result is always the same. 



The same feature is presented in the theory of elasticity, and in that 

 of capillarity. The neighboring molecules attract and repel each other, 

 we need not inquire by what law. It is enough for us that this at- 

 traction is sensible at small distances only, and that the molecules 

 are very numerous, that the medium is symmetrical, and we have 

 only to let the law of great numbers come into play. 



Here again the simplicity of the elementary phenomenon is hidden 

 beneath the complexity of the observable resultant phenomenon ; but in 

 its turn this simplicity was only apparent and disguised a very com- 

 plex mechanism. Evidently the best means of reaching the elementary 

 phenomenon would be experiment. It would be necessary by experi- 

 mental artifices to dissociate the complex system which nature offers 

 for our investigations and carefully to study the elements as disso- 

 ciated as possible; for example, natural white light would be decom- 

 posed into monochromatic lights by the aid of the prism, and into 

 polarized lights by the aid of the polarizer. Unfortunately, that is 

 neither always possible nor always sufficient, and sometimes the mind 

 must run ahead of experiment. I shall only give one example which 

 has always struck me rather forcibly. If I decompose white light, I 

 shall be able to isolate a portion of the spectrum, but however small it 

 may be, it will always be a certain width. In the same way the nat- 

 ural lights which are called monochromatic give us a very fine a ray, 

 but a ray which is not, however, infinitely fine. It might be supposed 

 that in the experimental study of the properties of these natural lights* 

 by operating with finer and finer rays, and passing on at last to the 

 limit, so to speak, we should eventually obtain the properties of a rigor- 

 ously monochromatic light. That would not be accurate. I assume 

 that two rays emanate from the same source, that they are first polar- 



