The wave-length of various kinds of monochromatic (one-colour) light can be easily determined by means of Young’s experiment on interference. If the distance between the two small holes from which the two streams of light emerge is measured, and if the distance from them to the screen and also the distance of the first dark band from the central line is determined, it is then very easy to calculate the difference in the distances from the two holes to the dark band. This difference, however, must, as already explained, be equal to one-half wave-length of the light employed. Experiments made in various ways have shown that the wave-length of yellow light is not far from the fifty thousandth part of an inch. Hence as the velocity of visible light is 186,500 miles per second, or 1000 million feet, or 12,000 million inches per second, whilst the wave-length is something like ¹⁄₅₀₀₀₀ inch, it is clear that the frequency, or number of light waves which enter the eye per second, must be reckoned in _millions of millions_. In fact, it ranges from 400 to 700 billions. There is a certain difference of opinion as to what is meant by a billion. We here use the word to signify a million times a million, a million being a thousand times a thousand. The following table shows us the frequency or number of waves per second, corresponding to light rays producing colour-sensations of various kinds:— VIBRATION RATES OF ÆTHER WAVES AFFECTING THE EYE AS LIGHT. Colour sensation. Vibrations per second. Deep red 400 billions. Red-orange 437 ” Yellow-orange 457 ” Yellow 509 ” Green 570 ” Blue-green 617 ” Blue-violet 696 ” Violet 750 ” Investigation has shown that the quality in a light ray which causes it to affect our eye with a particular colour-sensation is its wave-length, whereas the quality which affects our eyes as brightness or brilliancy is due to the amplitude of the waves. It is somewhat difficult to realize at first that, outside of ourselves, there is no such thing as colour. Colour is a sensation produced when æther waves of a certain wave-length enter the eye and fall on the retina. If the retina is stimulated 400 billions of times per second, we experience a sensation of redness, and if it is stimulated 700 billion times per second, we experience a sensation of blueness; but externally, there is no such thing as red and blue, there is only a difference in wave-frequency. It is astonishing when we learn for the first time that 400 millions of millions of times per second something in the back of our eyes is moved or stimulated whenever we look at a lady’s red dress, a surgeon’s red lamp, or the red petal of a geranium flower.