A vast body of proof has been accumulated that all these forms of radiation are merely varieties of one and the same thing, and that the only thing in which they really differ from one another is in what is called their wave-length. At this point I will remind you once more of that general law which connects together the velocity of propagation of a wave-motion, the wave-length and the frequency. It is expressed in the formula: wave-velocity (V) equals frequency (n) multiplied by wave-length (λ), or in symbolical language—
V = nλ
Accordingly, if the velocity of propagation can be determined, and if the frequency or periodicity of the wave-motion is known, then the wave-length can be found from the above simple rule; or conversely, if the velocity of propagation and the wave-length are known, the frequency is determined.
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.
You will notice, on referring to the above table of frequencies, that the range of sensibility of the human eye is very much smaller than that of the ear. Our eyes are wonderful instruments for detecting wave-motion in the æther, and our ears are appliances for detecting wave-motion in the air. The ear, however, is, as explained in a previous chapter, sensitive to air-vibration forming musical tones which lie between 30 and 30,000 per second, and these numbers are in the ratio of 1000 to 1, and cover a range of about ten octaves. The eye, however, is only sensitive to æther-vibrations which lie in frequency between 400 and 700 billions per second, and these numbers are in the ratio of nearly 2 to 1, or comprise only one octave.
The question, of course, immediately arises—What are the properties of æther waves the frequency of which lies outside the above limits?