THE WAVE THEORY OF LIGHT. 95 



that we owe the quickest and most perfect notions of the objects 

 around us: our other senses, hearing, feeling, also bring their share 

 of learning, but sight alone affords us abundant means of simultaneous 

 information such as no other sense can. It is, therefore, not surpris- 

 ing that light, this lasting link between us and the outward world, 

 should intervene with the varied sources of its inner constitution to 

 render more precise the observation of natural phenomena. Thus 

 each discovery concerning new properties of light has had an imme- 

 diate effect on the other branches of human knowledge, and has indeed 

 determined the birth of new sciences by affording new means of in\'es- 

 tigation of imexpected power and delicacy. 



Optics are really a modern science. The ancient philosophers had 

 no idea of the complexity of what is vulgarly called light; the}^ con- 

 founded in the same name what is proper to man, and what is exterior. 

 They had, however, perceived one of the characteristic properties of 

 the link, which exists between the source of light and the eye, which 

 receives the impression, "Light moves in a straight line." Common 

 experience had revealed this axiom through the observation of the 

 shining trains that the sun throws across the skies, piercing misty 

 clouds, or penetrating into some dark space. Hence arises two empir- 

 ical notions — the definition of the ray of light, and that of the straight 

 line. The one became the basis of optics, the other that of geometry. 



Very little remains to us of the ancient books upon optics. Yet we 

 are aware that they knew the reflection of the luminous rays on polished 

 surfaces and the geometrical explanation of the images formed b}" 

 mirrors. 



We must wait many centuries until the scientific revival for a new 

 progress in optics (but then a very considerable one) opens the new 

 era; it is the invention of the telescope. 



The new era begins with Galileo, Boyle, and Descartes, the founders 

 of experimental philosophy. All devote their life to meditations on 

 light, colors, and forces. Galileo lays the base of mechanics, and with 

 the refracting telescope that of astro-physics. Bojde improves experi- 

 mentation. As to Descartes, he embraces with his penetrating mind 

 the whole of natural philosophy; he throws away the occult causes 

 admitted })y the scholastics, and proclaims as a principle that all phe- 

 nomena are governed by the laws of mechanics. In his system of the 

 universe, light plays a prominent part: ^ it is produced by the waves 

 excited in the subtle matter which, according to his view, pervades 

 space. This subtle matter (which represents what ^ve call to-day the 

 ether) is considered by him as formed of particl(\>^ in inmiediate con- 

 tact; it constitutes thus at the same time the vehicle of the foi-ce.s exist- 

 ing between the material bodies which are plunged in it. We recognize 

 the famous ''vortices of Descartes," sometimes admired, sometimes 



^Le Monde de M. Descartes, ou le Traits de la Lumiere (Paris, 1664). 



