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40 THE ADVANCE OF PHOTOGRAPHY
a million as many as the tone which is marked in music with a bar over the a,1 that is,
P
The small number of the colour-tones compared with the large number of musical tones is very striking. But the fact is, that, besides the seven visible colours, there exist invisible tones, which are both deeper and higher than the visible colours.
These invisible colour-tones are partly disclosed by the thermometer, which reveals the lower tones, and partly by substances sensitive to light. For it is remarkable that the colour-tones, which are higher than the violet, though invisible, have a powerful chemical effect.
We name the invisible tones of colour above violet, ultra-violet, and those below red, infra-red.
Refraction of Light. — In the common white light all the tones of colour are found together, and in combination they produce the effect of whiteness ; but if we wish to examine the tones of colour separately, we must part them, and this may be done by the help of a prism.
Any polished crown-glass prism causes flames seen through it to appear like a rainbow containing the primitive colours we have named above. This separation of the colours in the prism takes place by refraction.
If a ray of light passes from one transparent medium to another, it is deflected from its rectilinear direction, and this deflection is named refraction.
For example, if the ray a n (fig. 11) strikes a surface of water, it does not continue in its original direction a n, but in the direction n b. If through the point n, where
1 We may here remark that the tone a is not everywhere the same. The a of the Berlin Opera is the highest — it has 437 vibrations ; the Italian Opera at Paris only, on the contrary, 424 vibrations. We have adopted for the sake of simplicity a round number, 420.