460 On Points in a Programme of Physics. 



them down to our own time. Thus, Huyghens inaugurated 

 the modern theory of light with his fruitful theory of waves, 

 whereby we can foretell in an admirable manner the mecha- 

 nism of propagation. Foretell, it must be said, rather than 

 demonstrate ; for the undulatory theory is tinged with abstrac- 

 tion, involves more of geometry than of physics ; and we at 

 present limit our attention too much to the individual wave, 

 which is destitute of that interpenetration and mutual influ- 

 ence which reality requires. The periodicity observed by New- 

 ton in coloured rings, and his analysis of light by dispersion, 

 at first seemed contrary to Huyghens's theory, but have since 

 proved to be its harmonious support. Behind the gigantic 

 figure of Newton we observe Euler and Young, each of whom 

 fixed his mark on the embryonic evolution of modern thought. 

 The former established analogies between colour and sound, 

 and showed that both depend on undulatory periods ; the 

 latter established experimentally the existence of interferences. 

 Fresnel made yet further progress by tracing the modifications 

 which light undergoes in its transit through crystals, thus 

 assisting us to form some conception of the intimate structure 

 of bodies. 



In consequence 'of these investigations, and the results by 

 which they have been followed, we now have recourse to energy 

 without troubling ourselves theoretically as to its particular 

 form ; we can measure each result in accordance with the 

 principle of unification. Such a possession is all the more 

 noble when we bear in mind the colossal nature of the forces 

 to be dealt with. 



We seek, and we find, relations between all the forces now 

 known to us, and we discover that they all spring from, and 

 are contained in, one natural energy. Can the conditions and 

 the time be more favourable for advancing a step further in 

 this path, and tracing in all its fulness the solid unity of the 

 constitution of physics ? No other science is so well prepared 

 for this reform ; at least, no other indicates it. 



We see, lastly, in this movement that which, in my poor 

 judgment, raises it to a much higher and more splendid posi- 

 tion. We see in it forces sufficiently energetic to be indepen- 

 dent of hypotheses ; we hear of researches in which, at each 

 step, the word atom needs no longer be pronounced. To sum 

 up briefly, we may affirm that the conception of energy here 

 adduced is in perfect harmony with modern physics ; but 

 chemistry and mechanics have to be submitted to its sway. 

 The actual condition of the science of energy is a supreme and 

 instinctive aspiration after unity — not an abstract and vague 

 unity, but one based on determinations and details. How we 



