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The figures at which we arrive are truly extraordinary. The dimensions and rates of oscillations of the waves which produce the different colours of visible light have been measured and calculated with the greatest accuracy, and they are as follows:

Dimensions of Light-Waves.

Colours No. of waves in one inch No. of oscillations in one second Red 39,000 477,000,000,000,000 Orange 42,000 506,000,000,000,000 Yellow 44,000 535,000,000,000,000 Green 47,000 577,000,000,000,000 Blue 51,000 622,000,000,000,000 Indigo 54,000 658,000,000,000,000 Violet 57,000 699,000,000,000,000

The elasticity of this wonderful medium is even more extraordinary.

The rapidity with which wave-motion is transmitted depends, other things being equal, on the elasticity of the medium, which is proportional to the square of the velocity with which a wave travels through it. As the velocity of the sound-wave in air is about 1,100 feet in a second, and that of the light-wave about 192,000 miles in the same time, it follows that the velocity of the latter is about a million times greater than that of the former, and if the density of ether were the same as that of air, its elasticity must be about a million million times greater. But the elasticity is the same thing as the power of resisting compression, which in the case of air we know to be about 15 pounds to the square inch; so that the ether, if equally dense, would balance a pressure of 15 million million pounds to the square inch—that is, it would require a pressure of about 750 millions of tons to the square inch to condense ether to the density of air. On the other hand, its density, if any, must be so infinitesimally small that the earth moving through it in its orbit with a velocity of 1,100 miles a minute suffers no perceptible retardation.