MODERN SCIENCE. 217 



DIFFRACTION OF LIGHT; after experimenting on the diffraction 

 of light (really on the interference of light), he discovered that 

 light added to light produces darkness a principle which led 

 Young and Fresnel to arrive at the truth of the undulatory 

 theory ; advanced optics generally and prepared the way for 

 Newton. 



162084. Mariotte discovered the LAW OF COMPRESSI- 

 BILITY OF GASES, that is, the contraction or shrinkage of 

 gases in proportion to pressure. This is called Mariotte's- 

 law, and also Boyle's law, the latter having also discovered it. 

 (See Boyle.) Mariotte explained the chief laws of motion by- 

 experiments which made them clearer than their mere 

 enunciation. 



1629 95. Huygens invented the cycloidal PENDULUM, 

 and also PENDULUM CLOCKS (1658); invented the MICRO- 

 METER to measure the apparent diameter of planets ; im- 

 proved the barometer, with which he taught how to measure 

 the altitude of mountains. With rare genius Huygens gave 

 a scientific BASIS TO THE UNDULATORY THEORY OF LIGHT 

 (1678), rightly thinking " that we can explain light," as he 

 said, " only by supposing it to be a VIBRATION LIKE SOUND."" 

 He was THE FIRST modern scientist (Democritus's theory in- 

 volved as much) to suggest that entire space is filled up with 

 an invisible substance called " ETHER," at once elastic and 

 subtle, and that luminous bodies make it vibrate so that its 

 undulations will strike upon the eye, and give us the sensa- 

 tion called light. He pointed out that in ordinary refraction 

 the velocity of propagation of the wave is equal in all 

 directions, so that the front of an advancing wave is spherical 

 and "reaches equal distances in equal times" a wonderful 

 feat of scientific intellect. And his experiments on crystals- 

 prove that he verified his theory. What is especially remark- 

 able and shows he understood the significance of his doctrine, 

 is that he gave a satisfactory explanation of refraction by the 

 undulatory theory. He also explained the phenomenon of 

 DOUBLE REFRACTION (through Iceland spar*) to be due to 

 the greater elasticity of this crystal in one direction than ii> 



* A crystal first made known by Bertholinus observing through it (irk 

 1660) the splitting of a light-ray into two rays. 



